Migrate to upstream workspace/textDocumentContent

[fwcd]

Sibling PR to

• Rename getReferenceDocument request to textDocumentContent sourcekit-lsp#1639

The workspace/textDocumentContent API will be upstreamed as part of the upcoming version 3.18 of LSP, thus we no longer need a non-standard workspace/getReferenceDocument implementation. Some notes on the implementation:

• Using this API requires upgrading to vscode-languageclient version 10, which is currently in beta. The migration requires using a different module resolution and updating some imports.
• Proposed features have to be enabled explicitly on the LanguageClient instance
• Similar to the upstreaming process of the inlay hint request, we keep the legacy request around for now and gate it behind a Swift version check (currently testing for toolchain version < 6.1, this might have to be updated depending on when the sourcekit-lsp PR is merged).

Since we may want to hold off with this until LSP 3.18 is finalized and the sourcekit-lsp PR is merged, I'm marking this PR as a draft for now. It already works, however, and can be used for testing.