Add support for break if

[JCapucho]

Adds support for the break if statement that may appear in the continuing block of a loop.

Relevant part of the spec: https://gpuweb.github.io/gpuweb/wgsl/#break-if-statement

Implements parsing in the wgsl frontend, emission on all the backends and validation for it.

Uniformity analysis isn't implemented, following the same behavior as regular breaks (this will be fixed in the uniformity analysis v5 rewrite).

All commits can be reviewed individually and are fully tested

Closes #1831

[JCapucho]

Nightly is changing the formatting of some parts, and if those changes are formatted with stable they are reverted

[jimblandy]

Thank you so much for 1) this PR and 2) leaving the SPIR-V front end change to a separate PR so my brain doesn't break.

This also needs some failure case coverage in tests/wgsl-errors.rs:

• misplaced break if statements
• non-bool expressions

A few suggestions, too.