Add C, C++, and Objective-C language support for sourcekit-lsp

[10000DOO]

Problem

In mixed Xcode projects (Swift + Objective-C), sourcekit-lsp only activates for .swift files because this extension only declares Swift as a supported language.

This means:

• .m / .mm / .h files get no LSP support from sourcekit-lsp
• Cross-references between Swift and ObjC code don't work (e.g. jump-to-definition from Swift → ObjC class fails)

Solution

sourcekit-lsp already handles C, C++, Objective-C, and Objective-C++ internally via its embedded clangd integration. The server is ready — Zed just isn't routing those file types to it.

This PR:

1. Migrates from the deprecated language field to the languages array
2. Adds Objective-C and Objective-C++ language definitions with tree-sitter-objc grammar
3. Routes all 5 languages to sourcekit-lsp

This matches how the official VSCode Swift extension already works:
https://github.com/swiftlang/vscode-swift/blob/fd00b062831a3919b07071b8b5f5e6a5690c6e8a/src/configuration.ts#L153-L157

Also, xcode-build-server already generates buildServer.json with all 5 languages declared — so the server side is already set up for this.

Changes

extension.toml:

# Before (deprecated single-language field)
[language_servers.sourcekit-lsp]
language = "Swift"

# After
[language_servers.sourcekit-lsp]
languages = ["Swift", "C", "C++", "Objective-C", "Objective-C++"]

[language_servers.sourcekit-lsp.language_ids]
Swift = "swift"
C = "c"
"C++" = "cpp"
"Objective-C" = "objective-c"
"Objective-C++" = "objective-cpp"

[grammars.objc]
repository = "https://github.com/amaanq/tree-sitter-objc"
commit = "181a81b8f23a2d593e7ab4259981f50122909fda"

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