Replace async-watch with a custom watch (#32245)

The `async-watch` crate doesn't seem to be maintained and we noticed
several panics coming from it, such as:

```
[bug] failed to observe change after notificaton.
zed::reliability::init_panic_hook::{{closure}}::hea8cdcb6299fad6b+154543526
std::panicking::rust_panic_with_hook::h33b18b24045abff4+127578547
std::panicking::begin_panic_handler::{{closure}}::hf8313cc2fd0126bc+127577770
std::sys::backtrace::__rust_end_short_backtrace::h57fe07c8aea5c98a+127571385
__rustc[95feac21a9532783]::rust_begin_unwind+127576909
core::panicking::panic_fmt::hd54fb667be51beea+9433328
core::option::expect_failed::h8456634a3dada3e4+9433291
assistant_tools::edit_agent::EditAgent::apply_edit_chunks::{{closure}}::habe2e1a32b267fd4+26921553
gpui::app::async_context::AsyncApp::spawn::{{closure}}::h12f5f25757f572ea+25923441
async_task::raw::RawTask<F,T,S,M>::run::h3cca0d402690ccba+25186815
<gpui::platform::linux::x11::client::X11Client as gpui::platform::linux::platform::LinuxClient>::run::h26264aefbcfbc14b+73961666
gpui::platform::linux::platform::<impl gpui::platform::Platform for P>::run::hb12dcd4abad715b5+73562509
gpui::app::Application::run::h0f936a5f855a3f9f+150676820
zed::main::ha17f9a25fe257d35+154788471
std::sys::backtrace::__rust_begin_short_backtrace::h1edd02429370b2bd+154624579
std::rt::lang_start::{{closure}}::h3d2e300f10059b0a+154264777
std::rt::lang_start_internal::h418648f91f5be3a1+127502049
main+154806636
__libc_start_main+46051972301573
_start+12358494
```

I didn't find an executor-agnostic watch crate that was well maintained
(we already tried postage and async-watch), so decided to implement it
our own version.

Release Notes:

- Fixed a panic that could sometimes occur when the agent performed
edits.

Cargo.lock

@@ -60,7 +60,6 @@ dependencies = [
  "assistant_slash_commands",
  "assistant_tool",
  "assistant_tools",
- "async-watch",
  "audio",
  "buffer_diff",
  "chrono",
@@ -131,6 +130,7 @@ dependencies = [
  "urlencoding",
  "util",
  "uuid",
+ "watch",
  "workspace",
  "workspace-hack",
  "zed_actions",
@@ -631,7 +631,6 @@ name = "assistant_tool"
 version = "0.1.0"
 dependencies = [
  "anyhow",
- "async-watch",
  "buffer_diff",
  "clock",
  "collections",
@@ -653,6 +652,7 @@ dependencies = [
  "settings",
  "text",
  "util",
+ "watch",
  "workspace",
  "workspace-hack",
  "zlog",
@@ -665,7 +665,6 @@ dependencies = [
  "agent_settings",
  "anyhow",
  "assistant_tool",
- "async-watch",
  "buffer_diff",
  "chrono",
  "client",
@@ -716,6 +715,7 @@ dependencies = [
  "ui",
  "unindent",
  "util",
+ "watch",
  "web_search",
  "which 6.0.3",
  "workspace",
@@ -1074,15 +1074,6 @@ dependencies = [
  "tungstenite 0.26.2",
 ]
 
-[[package]]
-name = "async-watch"
-version = "0.3.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "a078faf4e27c0c6cc0efb20e5da59dcccc04968ebf2801d8e0b2195124cdcdb2"
-dependencies = [
- "event-listener 2.5.3",
-]
-
 [[package]]
 name = "async_zip"
 version = "0.0.17"
@@ -5013,7 +5004,6 @@ dependencies = [
  "assistant_tool",
  "assistant_tools",
  "async-trait",
- "async-watch",
  "buffer_diff",
  "chrono",
  "clap",
@@ -5055,6 +5045,7 @@ dependencies = [
  "unindent",
  "util",
  "uuid",
+ "watch",
  "workspace-hack",
  "zed_llm_client",
 ]
@@ -8739,7 +8730,6 @@ version = "0.1.0"
 dependencies = [
  "anyhow",
  "async-trait",
- "async-watch",
  "clock",
  "collections",
  "ctor",
@@ -8789,6 +8779,7 @@ dependencies = [
  "unicase",
  "unindent",
  "util",
+ "watch",
  "workspace-hack",
  "zlog",
 ]
@@ -10147,7 +10138,6 @@ dependencies = [
  "async-std",
  "async-tar",
  "async-trait",
- "async-watch",
  "futures 0.3.31",
  "http_client",
  "log",
@@ -10157,6 +10147,7 @@ dependencies = [
  "serde_json",
  "smol",
  "util",
+ "watch",
  "which 6.0.3",
  "workspace-hack",
 ]
@@ -13007,7 +12998,6 @@ dependencies = [
  "askpass",
  "assistant_tool",
  "assistant_tools",
- "async-watch",
  "backtrace",
  "cargo_toml",
  "chrono",
@@ -13054,6 +13044,7 @@ dependencies = [
  "toml 0.8.20",
  "unindent",
  "util",
+ "watch",
  "worktree",
  "zlog",
 ]
@@ -17915,6 +17906,19 @@ dependencies = [
  "leb128",
 ]
 
+[[package]]
+name = "watch"
+version = "0.1.0"
+dependencies = [
+ "ctor",
+ "futures 0.3.31",
+ "gpui",
+ "parking_lot",
+ "rand 0.8.5",
+ "workspace-hack",
+ "zlog",
+]
+
 [[package]]
 name = "wayland-backend"
 version = "0.3.8"
@@ -19726,7 +19730,6 @@ dependencies = [
  "assistant_context_editor",
  "assistant_tool",
  "assistant_tools",
- "async-watch",
  "audio",
  "auto_update",
  "auto_update_ui",
@@ -19843,6 +19846,7 @@ dependencies = [
  "uuid",
  "vim",
  "vim_mode_setting",
+ "watch",
  "web_search",
  "web_search_providers",
  "welcome",