Add support for .mts and .cts config files

[ErlendS]

Related to https://github.com/tailwindlabs/tailwindcss/pull/13940

Now that tailwind supports .cts and .mts files, tailwindcss-intellisense should also look for them.

Added test where I thought it was appropriate. Note - v4 tests fail for me (but they do in the master branch has well)

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Context: I recently upgraded to node v22 in my private project and noticed some warnings that lead me to this tailwind PR. I tested switching tailwind.config to be .mts and noticed intellisense had stopped working.

Decided to try to help out by creating a PR that adds support for .cts and .mts files now that tailwind supports them.