Enable CSS-like linear gradient fills for flowchart nodes

[enourbakhsh]

📑 Summary

This PR adds support for CSS-like linear-gradient fills to Mermaid, following the CSS linear-gradient syntax. It introduces a reusable gradient utility that can be applied to other diagrams in the future, though this PR focuses on flowcharts as an example of how the feature can be used.

Resolves #2907 and #4476

📏 Design Decisions

The implementation involves updating the lexer to recognize the linear-gradient syntax and modifying the renderer to correctly apply the gradient fills to flowchart nodes. The design follows a CSS-like approach to maintain familiarity for users.

📋 Tasks

Make sure you

• 📖 have read the contribution guidelines
• 💻 have added necessary unit/e2e tests.
• 📓 have added documentation. Make sure MERMAID_RELEASE_VERSION is used for all new features.
• 🦋 If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running pnpm changeset and following the prompts. Changesets that add features should be minor and those that fix bugs should be patch. Please prefix changeset messages with feat:, fix:, or chore:.

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Build	Status	Details	Updated (UTC)
default (Inspect)	⚠️ Changes detected (Review)	339 added, 339 removed, 1 failure	Nov 1, 2024, 10:02 AM

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 821 lines in your changes missing coverage. Please review.

Project coverage is 4.43%. Comparing base (ece3b25) to head (df3e5b4).

Files with missing lines	Patch %	Lines
...kages/mermaid/src/rendering-util/createGradient.ts	0.00%	729 Missing ⚠️
.../src/diagrams/flowchart/flowRenderer-v3-unified.ts	0.00%	92 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           develop   #5913      +/-   ##
==========================================
- Coverage     4.50%   4.43%   -0.07%     
==========================================
  Files          383     385       +2     
  Lines        53875   54704     +829     
  Branches       622     597      -25     
==========================================
  Hits          2425    2425              
- Misses       51450   52279     +829     

Flag	Coverage Δ
unit	4.43% <0.00%> (-0.07%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
.../src/diagrams/flowchart/flowRenderer-v3-unified.ts	0.00% <0.00%> (ø)
...kages/mermaid/src/rendering-util/createGradient.ts	0.00% <0.00%> (ø)

... and 1 file with indirect coverage changes