extensions: Add support for snippets provided by extensions (#14020)

For now extensions can only register global snippets, but there'll be
follow-up work to support scope attribute in snippets.json.

Release Notes:

- Extensions can now provide snippets by including `snippets.json` file
next to the extension manifest.

---------

Co-authored-by: Marshall <marshall@zed.dev>
Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>

crates/snippet_provider/src/registry.rs

@@ -0,0 +1,53 @@
+use std::{path::Path, sync::Arc};
+
+use anyhow::Result;
+use collections::HashMap;
+use gpui::{AppContext, Global, ReadGlobal, UpdateGlobal};
+use parking_lot::RwLock;
+
+use crate::{file_stem_to_key, Snippet, SnippetKind};
+
+struct GlobalSnippetRegistry(Arc<SnippetRegistry>);
+
+impl Global for GlobalSnippetRegistry {}
+
+#[derive(Default)]
+pub struct SnippetRegistry {
+    snippets: RwLock<HashMap<SnippetKind, Vec<Arc<Snippet>>>>,
+}
+
+impl SnippetRegistry {
+    pub fn global(cx: &AppContext) -> Arc<Self> {
+        GlobalSnippetRegistry::global(cx).0.clone()
+    }
+
+    pub fn try_global(cx: &AppContext) -> Option<Arc<Self>> {
+        cx.try_global::<GlobalSnippetRegistry>()
+            .map(|registry| registry.0.clone())
+    }
+
+    pub fn init_global(cx: &mut AppContext) {
+        GlobalSnippetRegistry::set_global(cx, GlobalSnippetRegistry(Arc::new(Self::new())))
+    }
+
+    pub fn new() -> Self {
+        Self {
+            snippets: RwLock::new(HashMap::default()),
+        }
+    }
+
+    pub fn register_snippets(&self, file_path: &Path, contents: &str) -> Result<()> {
+        let snippets_in_file: crate::format::VSSnippetsFile = serde_json::from_str(contents)?;
+        let kind = file_path
+            .file_stem()
+            .and_then(|stem| stem.to_str().and_then(file_stem_to_key));
+        let snippets = crate::file_to_snippets(snippets_in_file);
+        self.snippets.write().insert(kind, snippets);
+
+        Ok(())
+    }
+
+    pub fn get_snippets(&self, kind: &SnippetKind) -> Vec<Arc<Snippet>> {
+        self.snippets.read().get(kind).cloned().unwrap_or_default()
+    }
+}