Handle legacy key behavior in theme-driven suggestions for @utility

[thecrypticace]

Fixes tailwindlabs/tailwindcss-intellisense#1328

The alpha and beta releases used _ in theme keys to represent a .. This meant we used --leading-1_5 instead of --leading-1\.5 to add utilities like leading-1.5. We prefer the use of the escaped dot now but still want to make sure suggestions for the legacy key format still works as expected when surrounded by numbers.

This is the same as #16433 but for @utility since we apparently missed this when emitting suggestions for it

[philipp-spiess]

Makes sense to make this behave the same as the default spacing utilities but I wonder when we can remove that special-casing for underscore separators like --spacing-1_5 😬

Do you think this (=the special handling for _) can eventually be moved into the IntelliSense extensions? 🤔

[thecrypticace]

@philipp-spiess oh… hmmm. I don't know. The main problem I see with moving it into IntelliSense is that we really only want this behavior for theme keys.

If someone were to define a utility like this:

@utility foo-2_5 { color: red; }

IntelliSense wouldn't know enough to ignore the replacement for this one utility so it would get suggested as foo-2.5 which would not work.

[thecrypticace]

It would be nice to remove this behavior though since it was really only for backwards compatibility with the alpha/beta releases. Maybe this is something we can discuss and figure out a timeline if/when we can remove it.

[philipp-spiess]

Yep totally makes sense. I wonder if in the future it would be helpful if a suggestion derrived from a theme key can also emit the used theme key variable somehow - could be fun for go-to-source or so or to find out when using a deprecated theme key 🤔