Move ELK to standalone package

[sidharthv96]

📑 Summary

Splits Flowchart-ELK into a separate package, to make licensing simpler.

Resolves #5043

This allows sites to fallback to dagre layout for flowchart-elk if the elk package is not installed, instead of completely breaking rendering.
We need to pick either #5050 or #5051 for the approach to override.

📏 Design Decisions

Moved files to a new package, removed elkjs dependency from the mermaid core.
Edited tsconfig to allow import of files from mermaid, to avoid duplicating code.

📋 Tasks

Make sure you

• 📖 have read the contribution guidelines
• 💻 have added necessary unit/e2e tests.

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[codecov]

Codecov Report

Merging #5049 (bcdd1c5) into next (b48136d) will decrease coverage by 2.21%.
Report is 577 commits behind head on next.
The diff coverage is 40.50%.

❗ Current head bcdd1c5 differs from pull request most recent head 5eb1160. Consider uploading reports for the commit 5eb1160 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             next    #5049      +/-   ##
==========================================
- Coverage   46.57%   44.37%   -2.21%     
==========================================
  Files          55       25      -30     
  Lines        6819     5217    -1602     
  Branches       36       25      -11     
==========================================
- Hits         3176     2315     -861     
+ Misses       3642     2901     -741     
  Partials        1        1              

Flag	Coverage Δ
unit	44.37% <40.50%> (-2.21%)	⬇️

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

Files	Coverage Δ
packages/mermaid/src/config.ts	52.22% <100.00%> (-8.10%)	⬇️
packages/mermaid/src/defaultConfig.ts	45.05% <100.00%> (+0.81%)	⬆️
packages/mermaid/src/diagram-api/detectType.ts	67.07% <0.00%> (+0.80%)	⬆️
packages/mermaid/src/diagrams/gantt/ganttDb.js	76.67% <60.00%> (+0.07%)	⬆️
packages/mermaid/src/diagrams/common/commonDb.ts	87.50% <87.50%> (ø)
packages/mermaid/src/diagram-api/diagramAPI.ts	58.82% <43.47%> (+3.12%)	⬆️
packages/mermaid/src/diagrams/common/common.ts	52.59% <19.78%> (-11.56%)	⬇️

... and 38 files with indirect coverage changes

[aloisklink]

I really like this approach!

The only thing I'd like to see if some E2E tests to make sure that flowchart-elk still renders correctly on the base Mermaid and when import flowchartELK from './mermaid-flowchart-elk.esm.mjs'; is run