Feature request: rustfmt current block.

[crlf0710]

Currently rustfmt does nothing to code within MBE. Before fully solving this problem. It would be nice if there's a workaround command that we can recognize the current block syntax the cursor is within, and reformat the code within the block with some recognized indentation.

[kjeremy]

There is a range version of the format request for LSP but we do not implement it since rustfmt is unreliable for document ranges.

[matklad]

This is a desirable feature, but it'll mostly fall out naturally after shaving this two large yaks: #4182, #1665