Support nested block comments

[dannymcgee]

This PR overhauls comment handling to support nested block comments. This was done by wrapping Gramatika's out-of-the-box ParseStream with an implementation that intercepts all peek, check and consume calls to check for comment-start tokens.

Comment nodes are stored in a separate buffer on this wrapper type that can be accessed by calling wgsl_parser::ParseStream::into_inner after parsing the syntax tree.

To accommodate error-handling while trying to parse comments, parse errors have also been moved to a separate buffer, which is also available on the type returned from into_inner.

Breaking Changes

• Previously, the following two lines of code were equivalent:

  let tree: SyntaxTree = gramatika::ParseStream::from(input).parse()?;
  let tree: SyntaxTree = wgsl_parser::ParseStream::from(input).parse()?;

  That's because wgsl_parser::ParseStream was just a type alias. That is no longer the case, so import paths may need to be updated to ensure ParseStream is being imported from wgsl_parser, not gramatika.

• ParseStream::into_inner() now returns a ParseResult instead of a tuple
• Token::BlockComment and Token::LineComment variants have been removed
• Token::CommentStart and Token::CommentEnd variants have been added
• Decl::Error variant has been removed
• Stmt::Error variant has been removed
• Expr::Error variant has been removed