Use updated R language syntax

[kylebutts]

There is now a new preferred language syntax for the R language (posit-dev/positron#5017). Given this produces better quality highlighting and is much more highly maintained than the previous one, I suggest changing to this one.

E.g. before, function calls were inconsistently highlighted:
Before:

After:

Before:

After:

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[antfu]

I am concerned about the license, I think it's a bit tricky as it's no longer a no-brainer to bundle and redistribute

[kylebutts]

@lionel- I'm wondering if there's a way to use the updated syntax. Shiki uses vscode grammars to produce high-quality code highlighting for html output. Perhaps, we could move the updated syntax .json back to the vscode-R repository?