python: Improve function syntax highlighting

[clwainwright]

Release Notes:

• Differentiate between function and method calls and definitions. function.definition matches the highlight for e.g. rust, function.call is new.
• Likewise differentiate between class calls and class definitions.
• Better highlighting of function decorators (the @ symbol is punctuation, and now the decorator itself has a function.decorator tag)
• Make cls a special variable (like self)
• Add ellipsis as a built-in constant

Note that most themes do not currently make use of the function.definition tags, and none make use of the type.class.definition tag. Hopefully more themes will pick this up.

Before:

After:

[cla-bot]

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[clwainwright]

@cla-bot check

[cla-bot]

The cla-bot has been summoned, and re-checked this pull request!

[osiewicz]

Thanks!