Detect and possibly use user-installed gopls / zls language servers (#8188)

After a lot of back-and-forth, this is a small attempt to implement
solutions (1) and (3) in
https://github.com/zed-industries/zed/issues/7902. The goal is to have a
minimal change that helps users get started with Zed, until we have
extensions ready.

Release Notes:

- Added detection of user-installed `gopls` to Go language server
adapter. If a user has `gopls` in `$PATH` when opening a worktree, it
will be used.
- Added detection of user-installed `zls` to Zig language server
adapter. If a user has `zls` in `$PATH` when opening a worktree, it will
be used.

Example:

I don't have `go` installed globally, but I do have `gopls`:

```
~ $ which go
go not found
~ $ which gopls
/Users/thorstenball/code/go/bin/gopls
```

But I do have `go` in a project's directory:

```
~/tmp/go-testing φ which go
/Users/thorstenball/.local/share/mise/installs/go/1.21.5/go/bin/go
~/tmp/go-testing φ which gopls
/Users/thorstenball/code/go/bin/gopls
```

With current Zed when I run `zed ~/tmp/go-testing`, I'd get the dreaded
error:

![screenshot-2024-02-23-11 14
08@2x](https://github.com/zed-industries/zed/assets/1185253/822ea59b-c63e-4102-a50e-75501cc4e0e3)

But with the changes in this PR, it works:

```
[2024-02-23T11:14:42+01:00 INFO  language::language_registry] starting language server "gopls", path: "/Users/thorstenball/tmp/go-testing", id: 1
[2024-02-23T11:14:42+01:00 INFO  language::language_registry] found user-installed language server for Go. path: "/Users/thorstenball/code/go/bin/gopls", arguments: ["-mode=stdio"]
[2024-02-23T11:14:42+01:00 INFO  lsp] starting language server. binary path: "/Users/thorstenball/code/go/bin/gopls", working directory: "/Users/thorstenball/tmp/go-testing", args: ["-mode=stdio"]
```

---------

Co-authored-by: Antonio <antonio@zed.dev>

crates/language/src/language.rs

@@ -38,6 +38,7 @@ use serde_json::Value;
 use std::{
     any::Any,
     cell::RefCell,
+    ffi::OsString,
     fmt::Debug,
     hash::Hash,
     mem,
@@ -140,6 +141,14 @@ impl CachedLspAdapter {
         })
     }
 
+    pub fn check_if_user_installed(
+        &self,
+        delegate: &Arc<dyn LspAdapterDelegate>,
+        cx: &mut AsyncAppContext,
+    ) -> Option<Task<Option<LanguageServerBinary>>> {
+        self.adapter.check_if_user_installed(delegate, cx)
+    }
+
     pub async fn fetch_latest_server_version(
         &self,
         delegate: &dyn LspAdapterDelegate,
@@ -240,6 +249,11 @@ impl CachedLspAdapter {
 pub trait LspAdapterDelegate: Send + Sync {
     fn show_notification(&self, message: &str, cx: &mut AppContext);
     fn http_client(&self) -> Arc<dyn HttpClient>;
+    fn which_command(
+        &self,
+        command: OsString,
+        cx: &AppContext,
+    ) -> Task<Option<(PathBuf, HashMap<String, String>)>>;
 }
 
 #[async_trait]
@@ -248,6 +262,14 @@ pub trait LspAdapter: 'static + Send + Sync {
 
     fn short_name(&self) -> &'static str;
 
+    fn check_if_user_installed(
+        &self,
+        _: &Arc<dyn LspAdapterDelegate>,
+        _: &mut AsyncAppContext,
+    ) -> Option<Task<Option<LanguageServerBinary>>> {
+        None
+    }
+
     async fn fetch_latest_server_version(
         &self,
         delegate: &dyn LspAdapterDelegate,