Update sentinel syntax highlighting

[glennsarti]

This PR updates the Sentinel Syntax Highlighting:

• Moves to an easier to understand "repository" style location for types of matches. This is similar to the TF syntax
• Updates the inbuilt function matching to NOT highlight non-function call locations e.g.

append(foo, bar)    <-- Highlight "append"
append = 2          <-- Do not highlight as it's not a function call

• Adds missing builtin function compare
• Fixes highlighting double symbol operators e.g. ==
• Highlights triple and double word operators better e.g. is not empty

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Note this PR should be squash merged

[glennsarti]

This is not everything, but it's a big enough chunk for a single PR.

[jpogran]

🚀