Stabilise --emit=checkstyle

--emit=checkstyle has existed in rustfmt for a number of years. Although there have been some bugs fixed in the past, nothing has been mentioned in the last year or so when it was suggested checkstyle/json were considered stable: https://github.com/rust-lang/rustfmt/issues/3947#issuecomment-561464005 I believe using these features still requires a nightly build of rustfmt: https://github.com/rust-lang/rustfmt/blob/rustfmt-1.4.31/src/bin/main.rs#L496

This feature is useful for integrating rustfmt into existing code lint pipelines which support checkstyle from other languages. It would be helpful to me to have this support in a stable rust release to avoid managing a nightly build for this feature.

I can't see any documentation or issues which describe why some emit options are behind a nightly release, so opening this to make progress or at least document the current state.

Two questions to start with:

1. Is there a path forward for these features to become available in stable builds of rustfmt & therefore stable builds of rust/cargo/etc?
2. Would a change to stablise this on a pre-2.0 feature branch be accepted? An untested/draft patch

PS Thank you for the effort put into this project. I can see there's a near constant stream of issues/bug reports here. I'm hoping this doesn't just add to the noise.

Start with rustfmt-1.4.31/src/bin/main.rs around line 496 and review the linked rustfmt issue discussion about checkstyle/json stability. Compare the referenced draft patch to the current project state. Done means reaching and documenting a clear decision on stable availability, or implementing and validating the accepted stabilization path.