Add crash stats output to git ignore (#19369)
Follow-up #19175
### Summary of the issue:
If NVDA crashes when running from source, git picks up
`nvda_crash_stats.txt` as an untracked file.
### Description of user facing changes:
None
### Description of developer facing changes:
Git will ignore `nvda_crash_stats.txt`, meaning it's much less likely to
be accidentally committed and doesn't clutter our git output.
### Description of development approach:
Added `source/nvda_crash_stats.txt` to `.gitignore`.
### Testing strategy:
Ran `git status`. Verified that `nvda_crash_stats` was no longer showing
as an untracked file.
### Known issues with pull request:
None
