Highlighting of multi-line `if` fields

**Is your feature request related to a problem? Please describe.**
I often use a complex `if` field to enable smart skipping of the `push` event trigger.
(note that without the `${{ }}` the editor yields a syntax error saying `Unexpected symbol: '=='`. Adding a space after the initial `!` seems to eliminate the error, but I'm not sure if that works (#290).)

Writing it like that looks horrible so I format it as a multi-line string, but then the syntax highlighting is broken.

**Describe the solution you'd like**
It would be really nice to have syntax highlighting for multi-line expressions. It would look something like this:

No file, test, or entry point is named in the issue. Start by locating the extension's syntax-highlighting implementation and any tests for GitHub Actions expressions, then compare current highlighting with the requested multi-line example. Done means multi-line expressions are highlighted correctly without breaking existing highlighting.

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