Move mathCATDir to ReadPaths (#19370)
Fixup #18323
### Summary of the issue:
`mathCATDir` is a property of `NVDAState.WritePaths`, even though it is
not writable by non-elevated installed copies, and it doesn't seem like
we even want to write to it.
### Description of user facing changes:
None
### Description of developer facing changes:
`mathCATDir` is now a property of `NVDAState.ReadDirs`.
### Description of development approach:
Moved `mathCATDir` to `NVDAState._ReadDirs`.
Searched for usages of `mathCATDir` in `source/` and updated them.
### Testing strategy:
Ran from source and interacted with MathML in Firefox.
### Known issues with pull request:
None
