feature: add basic page up/down scrolling

[ClementTsang]

Description

A description of the change and what it does. If relevant (such as any change that modifies the UI), please provide screenshots of the change:

Adds page up/down scrolling support to respectively scroll up/down by a full page.

Note that this is mostly just to get the feature out for those interested, and is admittedly a bit rushed - I will be rewriting all logic involving event handling as part of state refactor anyways, so this will also get changed in the work done there, and therefore, I kinda just sped through this.

Issue

If applicable, what issue does this address?

Closes: #620

Testing

If relevant, please state how this was tested. All changes must be tested to work:

Please also indicate which platforms were tested. All platforms directly affected by the change must be tested:

• Windows
• macOS
• Linux

Checklist

If relevant, ensure the following have been met:

• Areas your change affects have been linted using rustfmt (cargo fmt)
• The change has been tested and doesn't appear to cause any unintended breakage
• Documentation has been added/updated if needed (README.md, help menu, etc.)
• The pull request passes the provided CI pipeline
• There are no merge conflicts

[codecov-commenter]

Codecov Report

Merging #646 (a70b2f9) into master (eaff52c) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff            @@
##           master    #646      +/-   ##
=========================================
- Coverage    7.16%   7.14%   -0.03%     
=========================================
  Files          52      52              
  Lines        8285    8315      +30     
=========================================
  Hits          594     594              
- Misses       7691    7721      +30     

Impacted Files	Coverage Δ
src/app.rs	0.00% <0.00%> (ø)
src/canvas.rs	0.00% <0.00%> (ø)
src/constants.rs	0.00% <ø> (ø)
src/lib.rs	0.00% <0.00%> (ø)
src/utils/logging.rs	0.00% <ø> (ø)

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[ClementTsang]

Going to be ignoring codecov for this, this code is going to be dead in (hopefully) a few weeks anyways.