XMB Improvements, parse JS into XMB, make strings more efficient.

XMB format is bumped to 4, invalidating all cached files. The differences are:

• element/attribute names are stored after the elements themselves, and not before. This allows writing XMB data in one pass instead of two.
• names themselves becomes offsets (instead of arbitrary integers), making getting the string from the int name much more efficient.

XMBFile is renamed to XMBData to clarify that it does not, in fact, refer to a file on disk.

XMBData::GetElementString is also changed to return a const char*, thus not creating an std::string. A string_view version is added where convenient.

The XML->XMB and JS->XMB conversion functions and the corresponding storage are moved to ps/XMB, since that format doesn't particularly relate to XML. CXeromyces becomes lighter and more focused as a result.
The XML->XMB conversion also benefits from the above streamlining.

Note that in a few cases, string_view gets printed to CLogger via data(), which is generally not legal, but we know that the strings are null-terminated here. Our libfmt (version 4) doesn't support string_view, that would be v5.

Differential Revision: https://code.wildfiregames.com/D3909