Adding scroll bars with proper cell tags

[dprada]

Some lines were added to mystnb.css to add scroll bars with tags: 'output_scroll', 'scroll-output' and 'scroll-input'.

The lines implemented are similar to the paragraph in https://github.com/executablebooks/sphinx-book-theme/blob/eb1b6baf098b27605e8f2b7b2979b17ebf1b9540/src/sphinx_book_theme/assets/styles/extensions/_myst-nb.scss#L21. The only difference is the inclusion of indications for horizontal bars (width and y lines).

See #453

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[codecov[bot]]

Codecov Report

Base: 81.67% // Head: 81.63% // Decreases project coverage by -0.04% :warning:

Coverage data is based on head (0e94fca) compared to base (385c4a7). Patch has no changes to coverable lines.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #454      +/-   ##
==========================================
- Coverage   81.67%   81.63%   -0.04%     
==========================================
  Files          29       29              
  Lines        2647     2647              
==========================================
- Hits         2162     2161       -1     
- Misses        485      486       +1     

Flag	Coverage Δ
pytests	81.63% <ø> (-0.04%)	:arrow_down:

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Impacted Files	Coverage Δ
myst_nb/core/render.py	85.34% <0.00%> (-0.22%)	:arrow_down:

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