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!

[kfc35]

Another test case you could add if you wanted to is that it still writes the short type with two processors that are unique (not ambiguous), or you could just augment the existing test for writes_short_default_meta_for_processor with a second unique processor registered, but this suggestion can be ignored