File context for assistant panel (#9712)

Introducing the Active File Context portion of #9705. When someone is in
the assistant panel it now includes the active file as a system message
on send while showing them a nice little display in the lower right:


![image](https://github.com/zed-industries/zed/assets/836375/9abc56e0-e8f2-45ee-9e7e-b83b28b483ea)

For this iteration, I'd love to see the following before we land this:

* [x] Toggle-able context - user should be able to disable sending this
context
* [x] Show nothing if there is no context coming in
* [x] Update token count as we change items
* [x] Listen for a more finely scoped event for when the active item
changes
* [x] Create a global for pulling a file icon based on a path. Zed's
main way to do this is nested within project panel's `FileAssociation`s.
* [x] Get the code fence name for a Language for the system prompt
* [x] Update the token count when the buffer content changes

I'm seeing this PR as the foundation for providing other kinds of
context -- diagnostic summaries, failing tests, additional files, etc.

Release Notes:

- Added file context to assistant chat panel
([#9705](https://github.com/zed-industries/zed/issues/9705)).

<img width="1558" alt="image"
src="https://github.com/zed-industries/zed/assets/836375/86eb7e50-3e28-4754-9c3f-895be588616d">

---------

Co-authored-by: Conrad Irwin <conrad@zed.dev>
Co-authored-by: Nathan <nathan@zed.dev>
Co-authored-by: Antonio Scandurra <me@as-cii.com>
Co-authored-by: Mikayla Maki <mikayla@zed.dev>

crates/file_icons/Cargo.toml

@@ -0,0 +1,21 @@
+[package]
+name = "file_icons"
+version = "0.1.0"
+edition = "2021"
+publish = false
+license = "GPL-3.0-or-later"
+
+[lints]
+workspace = true
+
+[lib]
+path = "src/file_icons.rs"
+doctest = false
+
+[dependencies]
+gpui.workspace = true
+util.workspace = true
+serde.workspace = true
+serde_derive.workspace = true
+serde_json.workspace = true
+collections.workspace = true

crates/file_icons/src/file_icons.rs

@@ -0,0 +1,98 @@
+use std::{path::Path, str, sync::Arc};
+
+use collections::HashMap;
+
+use gpui::{AppContext, AssetSource, Global};
+use serde_derive::Deserialize;
+use util::{maybe, paths::PathExt};
+
+#[derive(Deserialize, Debug)]
+struct TypeConfig {
+    icon: Arc<str>,
+}
+
+#[derive(Deserialize, Debug)]
+pub struct FileIcons {
+    stems: HashMap<String, String>,
+    suffixes: HashMap<String, String>,
+    types: HashMap<String, TypeConfig>,
+}
+
+impl Global for FileIcons {}
+
+const COLLAPSED_DIRECTORY_TYPE: &str = "collapsed_folder";
+const EXPANDED_DIRECTORY_TYPE: &str = "expanded_folder";
+const COLLAPSED_CHEVRON_TYPE: &str = "collapsed_chevron";
+const EXPANDED_CHEVRON_TYPE: &str = "expanded_chevron";
+pub const FILE_TYPES_ASSET: &str = "icons/file_icons/file_types.json";
+
+pub fn init(assets: impl AssetSource, cx: &mut AppContext) {
+    cx.set_global(FileIcons::new(assets))
+}
+
+impl FileIcons {
+    pub fn new(assets: impl AssetSource) -> Self {
+        assets
+            .load("icons/file_icons/file_types.json")
+            .and_then(|file| {
+                serde_json::from_str::<FileIcons>(str::from_utf8(&file).unwrap())
+                    .map_err(Into::into)
+            })
+            .unwrap_or_else(|_| FileIcons {
+                stems: HashMap::default(),
+                suffixes: HashMap::default(),
+                types: HashMap::default(),
+            })
+    }
+
+    pub fn get_icon(path: &Path, cx: &AppContext) -> Option<Arc<str>> {
+        let this = cx.try_global::<Self>()?;
+
+        // FIXME: Associate a type with the languages and have the file's language
+        //        override these associations
+        maybe!({
+            let suffix = path.icon_stem_or_suffix()?;
+
+            if let Some(type_str) = this.stems.get(suffix) {
+                return this
+                    .types
+                    .get(type_str)
+                    .map(|type_config| type_config.icon.clone());
+            }
+
+            this.suffixes
+                .get(suffix)
+                .and_then(|type_str| this.types.get(type_str))
+                .map(|type_config| type_config.icon.clone())
+        })
+        .or_else(|| this.types.get("default").map(|config| config.icon.clone()))
+    }
+
+    pub fn get_folder_icon(expanded: bool, cx: &AppContext) -> Option<Arc<str>> {
+        let this = cx.try_global::<Self>()?;
+
+        let key = if expanded {
+            EXPANDED_DIRECTORY_TYPE
+        } else {
+            COLLAPSED_DIRECTORY_TYPE
+        };
+
+        this.types
+            .get(key)
+            .map(|type_config| type_config.icon.clone())
+    }
+
+    pub fn get_chevron_icon(expanded: bool, cx: &AppContext) -> Option<Arc<str>> {
+        let this = cx.try_global::<Self>()?;
+
+        let key = if expanded {
+            EXPANDED_CHEVRON_TYPE
+        } else {
+            COLLAPSED_CHEVRON_TYPE
+        };
+
+        this.types
+            .get(key)
+            .map(|type_config| type_config.icon.clone())
+    }
+}