Expands runtime editing to track changes to background and foreground textures

[ViMaSter]

Summary

Resolves #399. It's #349, but now with background and foreground sprites (and actors sandwiched in the middle).

Allows changing two sprites in-game without the need for the Unity Editor

Before/after screenshots and/or animated gif

Unity_yrIS2NnpiH.mp4

Testing instructions

Runtime editing usage

1. Open the main menu in the editor or start a pre-compiled binary of the game
2. Start the game normally
3. Press Ctrl+R
4. Notice the system's file browser opening a directory
5. Notice the game starting to play Script.ink inside the shown directory
6. Change Background.png or Foreground.png and save them
7. Notice the game automatically reloading
8. Run all tests
9. Check the updated wiki page

Additional information

• [ ] Changes UI
• [x] Introduces new feature
  • Runtime editing of sprites
• [ ] Removes existing feature
• [ ] Has associated resource:

[codecov-commenter]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (abc65be) 78.33% compared to head (3d49363) 78.50%.

Files	Patch %	Lines
...Assets/Scripts/RuntimeEditing/FileSystemWatcher.cs	94.49%	6 Missing ⚠️

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Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #401      +/-   ##
===========================================
+ Coverage    78.33%   78.50%   +0.16%     
===========================================
  Files           86       86              
  Lines         2959     2982      +23     
===========================================
+ Hits          2318     2341      +23     
  Misses         641      641              

Flag	Coverage Δ
EditModeTests_Suites_Core	15.76% <0.00%> (-0.13%)	⬇️
EditModeTests_Suites_TextDecoder	16.29% <0.00%> (-0.13%)	⬇️
PlayModeTests_Suites_Playthrough	59.55% <57.79%> (+0.34%)	⬆️
PlayModeTests_Suites_Scenes	56.67% <57.79%> (+0.37%)	⬆️
PlayModeTests_Suites_Scripts	57.10% <94.49%> (-0.52%)	⬇️

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