Deprecate process shell block

[bentsherman]

Based on some recent internal discussions. The original purpose of the shell block was to help distinguish between Nextflow and Bash variables in the process script. Since the VS Code extension now provides both syntax highlighting and error checking for variable names, the shell block isn't needed as much and can be phased out to promote consistent code.

I didn't bother to add a warning in the code because I will add one in the language server, but we can add one here if you want.

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[pditommaso]

It would make sense to add a deprecation warning (and a warning to disable it)