Make the `mod parser` public.

[sushoucanyuan]

Why isn't mod parser public?
I need to use the code from this module to implement the functionality.

[devongovett]

It's just internals. Why do you need it?

[sushoucanyuan]

I am trying to implement parsing of non-standard CSS syntax based on lightingcss.
Directly using StyleSheet::parse_with does not meet my requirements, so I am trying to implement a function similar to StyleSheet::parse_with and call a self-implemented TopLevelRuleParser within it. In the self-implemented TopLevelRuleParser, the parser::NestedRuleParser will be invoked.
If the mod parser is not public, I cannot reference AtRulePrelude and NestedRuleParser.