chore: remove circular dependency between blockly and dev-tools

[lizschwab]

The basics

• I validated my changes

The details

Resolves

Fixes

Proposed Changes

• Removes the Advanced Playground link from the playground
• Removes the Advanced Playground html file from the Blockly package
• Removes the magic_symlink script, which was only used for running the Advanced Playground directly in Blockly
• Removes dependency on @blockly/dev-tools and @blockly/theme-modern from blockly package (theme-modern is only needed for running the Advanced Playground)

Reason for Changes

We need to resolve the circular dependency to get Lerna and Nx to be able to automate build processes. As things are now, they cannot resolve build order because of the circular dependency.

Additional Information

This will change the workflow for running the advanced playground.

New Advanced Playground workflow:

1. Run npm run start from packages/plugins/dev-tools/
   - or -
   Run npm run start --workspace=@blockly/dev-tools from monorepo root
2. Make changes in blockly
3. From a separate terminal window, run npm run build from packages/blockly
   - or -
   Run npm run build --workspace=blockly from monorepo root