Fix class detection in Slim templates with attached attributes and IDs

[thecrypticace]

This PR brings back a tweaked version of our "inner matches" regex so the m*-auto classes in the following Slim template will be properly detected:

.ml-auto[
  data-value='foo'
]
  Foo bar
.mr-auto[data-value='foo']
  Foo bar
.mt-auto#omg
  Foo bar
#omg.mb-auto
  Foo bar

The regex isn't identical to the previous one because we still want to prevent things like px-1.5 from detecting both px-1 and px-1.5.

Fixes #14013

[ArnaudBarre]

Hello! Just wanted to let you know that by adding . to left border and #[ to right border in https://github.com/ArnaudBarre/downwind/blob/main/src/utils/regex.ts#L59-L63 this four cases passes. And I don't have any issues with px-1.5 after adding that this is a possible value (long live px based size config). I really love how lookahead and lookbehind is making it possible to get everything in one regex (I think one regex per file type would be ideal).

[thecrypticace]

Yeah regex lookbehind wasn't supported until Safari 16.4 annoyingly which is why we don't use it (b/c CDN and Play). Won't matter in the future though.