feat: implement textDocument/formatting

[steeve]

This is done by spawning a buildifier process.

Buildifier can be configured in the initialization options:

{
    "buildifier": {
        "path": "...",
        "args": ["...", "...."]
     }
}

Closes #267

PS: note that this was done at 99% by Gemini Pro 2.5

[steeve]

Gentle ping @withered-magic :)

[withered-magic]

sorry for late movement here, I'll try to get this in soon!