x/tools/gopls: use RelativePatterns for file watching if RelativePatternSupport is available

[findleyr]

In order to fix problems with Windows paths, we should use RelativePatterns if they are supported:
https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#relativePattern

Because these patterns allow specifying a directory URI, they are a better solution for watching absolute directories, and don't suffer from windows drive casing issues.

See microsoft/vscode#104387 for more background.

[findleyr]

This is definitely necessary: I discovered that e.g. absolute patterns do not work in coc.nvim.

[gopherbot]

Change https://go.dev/cl/557499 mentions this issue: gopls: use relative watched file patterns if supported