Provide minimal edits when formatting

[trixnz]

When formatting the whole file, we return a single text edit containing the whole formatted document, which can sometimes cause vscode to lose track of the cursor position. According to microsoft/vscode#10133, vscode prefers that extensions provide a minimal number of edits.

We should instead use the proposed --write-mode=diff argument to provide vscode with a minimal number of edits.

This has a dependency on trixnz/lua-fmt#13.