feat: Upgrade to Remix 2

[dcramer]

• Switch to standard functions for action/loader
• Flatten routes
• Various fixes for dependency upgrades

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 49.21466% with 97 lines in your changes are missing coverage. Please review.

Project coverage is 39.79%. Comparing base (4a23b07) to head (7f403d5).

Files	Patch %	Lines
app/routes/u.$userEmail._index.tsx	44.44%	10 Missing :warning:
app/routes/admin.tsx	50.00%	8 Missing :warning:
app/routes/p.$postId._index.tsx	33.33%	8 Missing :warning:
app/routes/settings.tsx	46.15%	7 Missing :warning:
app/routes/new-post.tsx	33.33%	6 Missing :warning:
app/routes/p.$postId_.edit.tsx	33.33%	6 Missing :warning:
app/models/user.server.ts	50.00%	5 Missing :warning:
app/routes/admin.categories.$categoryId.tsx	50.00%	5 Missing :warning:
app/routes/admin.categories.new.tsx	55.55%	4 Missing :warning:
app/routes/admin.feeds.$feedId.tsx	42.85%	4 Missing :warning:
... and 16 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #136 +/- ## ========================================== - Coverage 40.96% 39.79% -1.17% ========================================== Files 88 88 Lines 7622 7411 -211 Branches 305 307 +2 ========================================== - Hits 3122 2949 -173 + Misses 4500 4462 -38 ```

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