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.toml

@@ -165,6 +165,7 @@ members = [
     "crates/util_macros",
     "crates/vim",
     "crates/vim_mode_setting",
+    "crates/watch",
     "crates/web_search",
     "crates/web_search_providers",
     "crates/welcome",
@@ -373,6 +374,7 @@ util = { path = "crates/util" }
 util_macros = { path = "crates/util_macros" }
 vim = { path = "crates/vim" }
 vim_mode_setting = { path = "crates/vim_mode_setting" }
+watch = { path = "crates/watch" }
 web_search = { path = "crates/web_search" }
 web_search_providers = { path = "crates/web_search_providers" }
 welcome = { path = "crates/welcome" }
@@ -403,7 +405,6 @@ async-recursion = "1.0.0"
 async-tar = "0.5.0"
 async-trait = "0.1"
 async-tungstenite = "0.29.1"
-async-watch = "0.3.1"
 async_zip = { version = "0.0.17", features = ["deflate", "deflate64"] }
 aws-config = { version = "1.6.1", features = ["behavior-version-latest"] }
 aws-credential-types = { version = "1.2.2", features = [