Fix file watcher data race

[jakebailey]

Fixes #1163

No need for a mutex here, a SyncSet and atomics are correct for this given watchers are increasing and unique.

[Copilot]

Pull Request Overview

This PR fixes a file watcher data race by replacing non-thread-safe variables with atomic types and a thread-safe set implementation.

• Uses atomic.Uint32 for safely incrementing the watcher ID.
• Replaces collections.Set with collections.SyncSet for managing watcher handles.
• Introduces a new thread-safe set implementation in internal/collections/syncset.go.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
internal/lsp/server.go	Updated watcher management by using atomic operations and SyncSet.
internal/collections/syncset.go	New thread-safe set implementation for storing watcher handles.

Comments suppressed due to low confidence (1)

internal/lsp/server.go:199

• [nitpick] Consider renaming the local variable 'watcherId' to 'watcherID' to maintain consistency with the struct field name.

watcherId := fmt.Sprintf("watcher-%d", s.watcherID.Add(1))