Breaking: Refactor signatures (take 2) and other stuff

[savq]

Breaking changes:

• Remove short_function_definition
  • short functions are now parsed as (= (call …) …) similar to the actual julia parser.
• Add visible signature rule in function_definition and macro_definition
  • This acts as a wrapper around a call or an argument list (if the function is anonymous).
• Add unary_typed_expression
  • Like typed_expression, this is a separate rule to make it easier to highlight the RHS as a type, tho it could probably be improved further.
• Add _closed_assignment to handle assignments inside brackets, where they cannot contain open tuples.
• Rename argument_list to arguments.
• Rename bare_tuple to open_tuple.
• Rename relative_qualifier to import_path (the name used in JuliaSyntax).

Internal changes:

• Remove _quotable
• Remove _number
• Refactor _operation

Closes #88 Closes #98 Closes #113 Closes #119 Closes #129 Closes #125

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This PR is kind of messy, but most of these changes are ports of improvements I've done while rewriting the julia lezer grammar.