Mention style for new syntax in tracking issue template
`@rust-lang/style` would like the specification of new syntax in the style guide to be part of the feature stabilization process, in order to avoid situations where new syntax is stabilized and it never has style specified or formatting implemented for it. This most recently occurred with [let-else](https://blog.rust-lang.org/2023/07/01/rustfmt-supports-let-else-statements.html). We've made a lot of progress with the [nightly style procedure](https://github.com/rust-lang/style-team/blob/master/nightly-style-procedure.md) to unblock rustfmt from experimenting with formatting for new syntax, and T-style's existence means we actually have people who are willing and qualified to make decisions about formatting specification.
This check-box should also perhaps include "formatting support implemented in rustfmt", but that's really up to `@rust-lang/rustfmt,` so I'm not volunteering them for any new responsibilities in this PR just yet.
Putting this up mostly to discuss with T-lang, though feedback welcome from anyone.
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As more of an implementation detail: alternatively, instead of a this could be just added to the existing rustc-dev-guide chapter(s) on stabilization.
r? `@ghost`
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