Enable strict type checking

[philippjfr]

• Enforce mypy passing as a CI condition
• Fix mypy
• Add all optional dependencies to type checking env

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 78.19253% with 222 lines in your changes missing coverage. Please review.

Project coverage is 82.28%. Comparing base (a248460) to head (060f415). Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
panel/io/pyodide.py	0.00%	18 Missing :warning:
panel/io/state.py	80.64%	18 Missing :warning:
panel/io/resources.py	58.06%	13 Missing :warning:
panel/io/jupyter_server_extension.py	0.00%	12 Missing :warning:
panel/io/server.py	64.70%	12 Missing :warning:
panel/reactive.py	81.13%	10 Missing :warning:
panel/template/base.py	56.52%	10 Missing :warning:
panel/io/document.py	66.66%	8 Missing :warning:
panel/pane/base.py	63.63%	8 Missing :warning:
panel/io/jupyter_executor.py	0.00%	7 Missing :warning:
... and 42 more

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #7497 +/- ## ========================================== - Coverage 82.38% 82.28% -0.11% ========================================== Files 339 339 Lines 50995 51227 +232 ========================================== + Hits 42014 42152 +138 - Misses 8981 9075 +94 ```

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[hoxbro]

Not finished with the review.