dslx_fmt should support waivers

[hongted]

Sometimes, it is desirable to not auto format sections of code (ex. comments that should not be reflow'ed). This issue is to support such a mechanism.

[dplassgit]

After some discussion we settled on // dslx-fmt::off and // dslx-fmt::on to control the formatting for phase 1.

Phase 2 will allow an attribute, e.g., #[dslx-fmt::disabled] to be applied to a whole module, function, etc.

[dplassgit]

This is inserting an extra closing brace sometimes.

[dplassgit]

I'm going to close this because it's mostly working, and if you want to disable the disabling, just don't disable formatting.