Add Python type stub (.pyi) generation for tesseract_decoder

[dandragona-dev]

Addresses #154.

1. Creates python stub file generation script using pybind11-stubgen. This script stores the files under src/tesseract_decoder-stubs. Each .pyi file is auto-generated without any modifications.

2. Integrates this into CMake and GitHub Workflows. Note that I was not able to successfully cmake this project (even at head without any changes), so consider my changes to CmakeLists.txt as untested (should I just drop them?).

3. Adds tests to ensure stubfiles exist and are valid python (maybe we can drop the valid python check + symbol expectation, WDYT?). Note that I think there is an issue with the visualizer stubs since

Smaller changes:
a. Adds pybind11-stubgen to the requirements file.
b. Creates src/tesseract_decoder-stubs/ directory to house the .pyi files.
c. Adds comments to the pybind files telling devs to regenerate the stubfiles upon making pybind changes.

Verification:
Unit tests, and just seeing if autocomplete works now:
Autocomplete without stubfiles:

Autocomplete with stubfiles:

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