Add i18n-report mode to show diff between two sets of translations

[qwandor]

The report looks like this:

Counts are "translated (fuzzy, fuzzy untranslated) / total"
Removed bn: 752 (54, 1) / 3761, creation date 2024-03-03T20:37:57+05:30
Added fo: 568 (178, 1) / 3651, creation date 2024-01-24T13:24:49+01:00
Changed ro: 1032 (2731, 0) / 3763, creation date 2024-05-15T20:58:33+03:00 -> ro: 1018 (2731, 0) / 3749, creation date 2024-05-15T20:58:33+03:00

The intention is that this can be used from a GitHub action to show a summary of translation changes on PRs.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 98 lines in your changes missing coverage. Please review.

Project coverage is 84.73%. Comparing base (5a15cf5) to head (23075ab).

Files	Patch %	Lines
i18n-report/src/main.rs	0.00%	86 Missing :warning:
i18n-report/src/stats.rs	0.00%	12 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #210 +/- ## ========================================== - Coverage 86.99% 84.73% -2.26% ========================================== Files 15 15 Lines 3229 3315 +86 Branches 3229 3315 +86 ========================================== Hits 2809 2809 - Misses 327 413 +86 Partials 93 93 ```

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