settings: Fix VS Code import appending duplicate file associations

[IbrahimKhan12]

Closes #56536

What & why

Running zed: import vs code settings more than once kept re-adding the same file extensions to file_types, so files.associations entries grew without bound:

"file_types": { "c": ["*.keymap", "*.keymap", "*.keymap"] }

The import merges the VS Code-derived settings into the user's existing settings, and file_types values were ExtendingVecs, whose merge_from appends every incoming value unconditionally.

Per review feedback, instead of changing ExtendingVec's merge semantics (it also backs ssh/wsl connections, private_files, etc.), this adds a new ExtendingSet type that actually behaves like a set, and switches FileTypeMap's file associations to it:

• Backed by an insertion-order-preserving IndexSet, so users' pattern order round-trips through the settings file unchanged (the import machinery diffs serialized old/new content to produce edits; a sorted set would reorder existing arrays).
• Merging only accumulates new values, so re-importing an already-present association is a no-op.
• Since it's a real set, deserialization also collapses duplicates that earlier imports already wrote into the settings file.
• ExtendingVec and all its other users are untouched.

Testing

• Added a test_vscode_import case that re-imports an already-present files.associations entry and asserts the extension isn't duplicated. It fails before the change and passes after.
• cargo test -p settings -p settings_content -p language pass.
• ./script/clippy -p settings -p settings_content -p language -p recent_projects -p json_schema_store is clean.

Release Notes:

• Fixed importing VS Code settings repeatedly adding duplicate file_types entries for files.associations.

[ChristopherBiscardi]

The ExtendingVec is also used in a couple of other locations (such as in remote_connections.rs for ssh/wsl connections), so we need to make sure the change in semantics here doesn't have any adverse impact on those use cases

[ChristopherBiscardi]

I think we should actually create a new type (ExtendingSet) here that actually behaves like a Set (instead of just behaving like a Set when extending). Then we can use the new ExtendingSet where we want to handle the file associations and leave the wsl/ssh usages of ExtendingVec alone.

[IbrahimKhan12]

Ack, will look into it.

[IbrahimKhan12]

Updated to address feedback.

[ChristopherBiscardi]

thanks!