[WIP] Improve and extend mappings check command.

[modmuss50]

This PR is WIP, please suggest feedback and other possible checks. (Note this should not be checking yarn code styling, that would need to go else where, possibly by exposing the MappingCheck interface).

The package visablity checks have been made warnings as they shouldnt fail the build right now. I might expand the CLI to allow control over this.

Currently this PR adds checks for synthetic entries with names, and invalid entries. (Same as drop mappings)

[enbrain]

Are these checks in the scope of this PR?

• check for synthetic entries with javadoc
• check for unnamed methods without params or javadoc
• check for empty COMMENT
• check for file names mismatch with class names

[ramidzkh]

A check for the issues described in https://github.com/FabricMC/yarn/issues/1145 would be nice