fix(build): restore language server asset copy in watch:desktop

Description

A recent change (refactor(sandbox): Include sandbox_worker.js in... (!3172 - merged)) removed the copy-language-server-assets task from the watch:desktop script, which prevents the "Run Extension" launch task from working.

This change restores the copy task.

Related Issues

Resolves #2278 (closed)

How has this been tested?

1. Check out this branch.
2. Open VS Code and navigate to the Run and Debug tab.
3. Run the "Run Extension" task.

The language server should run with no errors.

• If src/browser or src/common has been modified, please consider interoperability with the Web IDE. See Running the Extension in WebIDE.
• Consider an end-to-end test for significant new features that aren't covered by integration tests.

Screenshots (if appropriate)

What CHANGELOG entry will this MR create?

• fix: Bug fix fixes - a user-facing issue in production - included in changelog
• feature: New feature - a user-facing change which adds functionality - included in changelog
• BREAKING CHANGE: (fix or feature that would cause existing functionality to change) - should bump major version, mentioned in the changelog
• None - other non-user-facing changes