Add experimental LSP-based context retrieval system for edit prediction (#44036)

To do

* [x] Default to no context retrieval. Allow opting in to LSP-based
retrieval via a setting (for users in `zeta2` feature flag)
* [x] Feed this context to models when enabled
* [x] Make the zeta2 context view work well with LSP retrieval
* [x] Add a UI for the setting (for feature-flagged users)
* [x] Ensure Zeta CLI `context` command is usable

---

* [ ] Filter out LSP definitions that are too large / entire files (e.g.
modules)
* [ ] Introduce timeouts
* [ ] Test with other LSPs
* [ ] Figure out hangs

Release Notes:

- N/A

---------

Co-authored-by: Ben Kunkle <ben@zed.dev>
Co-authored-by: Agus Zubiaga <agus@zed.dev>

crates/edit_prediction_context2/Cargo.toml

@@ -0,0 +1,42 @@
+[package]
+name = "edit_prediction_context2"
+version = "0.1.0"
+edition.workspace = true
+publish.workspace = true
+license = "GPL-3.0-or-later"
+
+[lints]
+workspace = true
+
+[lib]
+path = "src/edit_prediction_context2.rs"
+
+[dependencies]
+parking_lot.workspace = true
+anyhow.workspace = true
+collections.workspace = true
+futures.workspace = true
+gpui.workspace = true
+language.workspace = true
+lsp.workspace = true
+project.workspace = true
+log.workspace = true
+serde.workspace = true
+smallvec.workspace = true
+tree-sitter.workspace = true
+util.workspace = true
+
+[dev-dependencies]
+env_logger.workspace = true
+indoc.workspace = true
+futures.workspace = true
+gpui = { workspace = true, features = ["test-support"] }
+language = { workspace = true, features = ["test-support"] }
+lsp = { workspace = true, features = ["test-support"] }
+pretty_assertions.workspace = true
+project = {workspace= true, features = ["test-support"]}
+serde_json.workspace = true
+settings = {workspace= true, features = ["test-support"]}
+text = { workspace = true, features = ["test-support"] }
+util = { workspace = true, features = ["test-support"] }
+zlog.workspace = true