Make write_default_meta_file_for_path write the short type path instead of the long type path.

[andriyDev]

Objective

• Context: In Stop automatically generating meta files for assets while using asset processing. #17216, we stopped automatically generating meta files for assets, but provided explicit functions for generating meta files.
• In Allow using short type names for asset processors. #21339, we allowed asset processors to be specified by their short type path in meta files.
• However, the meta file generating functions still produced fully-qualified type paths for processors. So even though we supported much more readable processor meta files, we didn't take advantage of that when generating meta files.
• Fulfilling my obligations to cart: Allow using short type names for asset processors. #21339 (comment)

Solution

• Make ErasedProcessor have a method for returning its short_type_path, and make its default_meta method take an enum to choose between using the short or long type path.
• When generating the default meta, first lookup the processor using its short type name. If it's ambiguous, use the long type path.
• Add tests to show the short/long selection. As part of this, I forced all meta files to be serialized with \n as the new line character.
• Bonus: add more tests for write_default_meta_file_for_path and write_default_loader_meta_file_for_path which were omitted in Stop automatically generating meta files for assets while using asset processing. #17216 due to missing appropriate testing utilities (specifically a memory asset writer).

Testing

• Added tests!