feature: support 3-char hex colours

[ClementTsang]

Description

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

Support three-character hex colours (e.g. "#fff").

Issue

If applicable, what issue does this address?

Closes: #

Testing

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

If this is a code change, please also indicate which platforms were 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, doc pages, etc.)
• The pull request passes the provided CI pipeline
• There are no merge conflicts
• If relevant, new tests were added (don't worry too much about coverage)

[codecov]

Codecov Report

Base: 20.96% // Head: 21.31% // Increases project coverage by +0.34% 🎉

Coverage data is based on head (1339bc2) compared to base (edc61d4).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1022      +/-   ##
==========================================
+ Coverage   20.96%   21.31%   +0.34%     
==========================================
  Files          75       75              
  Lines       14694    14754      +60     
==========================================
+ Hits         3081     3145      +64     
+ Misses      11613    11609       -4     

Impacted Files	Coverage Δ
src/canvas/canvas_styling/colour_utils.rs	97.53% <100.00%> (+3.83%)	⬆️

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