Convert variable names to snake_case

[GearsDatapacks]

Closes #2978 Let me summarise what I've done:

• BadName errors are now no longer reported in the lexer. The lexer treats all previous bad names as valid.
• The analyser now is responsible for validating names. It checks the name of variables (let, use and const), functions, parameters, types and type constructors.
• Finding an error in a variable name still declares it, to allow any references to that variable to still type-check correctly.
• When the analyser finds an invalid name, it adds it to a bad_names array, just like unused_imports.
• I've had to add more location information to certain ast nodes to allow proper renaming of variables without overwriting other parts of the source code

The main problem I see with this approach is that it would be easy to miss validation of names in certain places, but I can't think of an easy way to check all names in the analyser. This implementation also doesn't rename any references to invalid variables, but #2956 may need to be closed before that's possible/easy to add.