[ux] Cross-platform compatibility for symlinking of demo/node_modules

[winstonwumbo]

New Features

• Removed the symlink at elements/haxcms-elements/demo/node_modules from version control
  • By default, GitHub Desktop disables any Git-tracked symlinks on a Windows machine.
  • There is not a GUI setting to change this. Instead users had to run git config --global core.symlinks true or manually edit $HOME/.gitconfig
• Added elements/haxcms-elements/demo/node_modules to gitignore
• Now the symlink is initialized in postinstall.js, running as another build step of yarn install
  • We bypass the GitHub Desktop limitation and further reduce HAX's configuration requirements
  • Tested on Linux, macOS, and Windows
• Updated README with clearer instructions for Dev Containers

[vercel]

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