Fix minor variable grammar bugs

[dolphinau]

Create a new token to match the variable names only, without the $, @ or ${...}

[dolphinau]

New commits:
Adding variable_name doesn't look possible because we cannot break the variable token
However, I still fixed few bugs and added new corner cases:

• $::
• Remove : after reserved keywords
• Add function and other PSDrive keywords
• Add potential : in the variable name (ex: $local:test:test:test is valid but not $local:test::test:test)
• Remove braced variable
• Add escaped } with backticks for braced variables