Move devtools out of platform

[rebornix]

Re #10152

This PR

• Moves devtools out of platform
• Move isPythonNotebook into kernels for now as this helper reads kernel metadata info

This is the last piece before we turn on all eslint rules and consolidate our typings protection (for telemetry, commands and webview message types, will discuss in standup). With this change, we have no layer breaking imports.

• Pull request represents a single change (i.e. not fixing disparate/unrelated things in a single PR).
• Title summarizes what is changing.
• Has a news entry file (remember to thank yourself!).
• Appropriate comments and documentation strings in the code.
• Has sufficient logging.
• Has telemetry for feature-requests.
• Unit tests & system/integration tests are added/updated.
• Test plan is updated as appropriate.
• package-lock.json has been regenerated by running npm install (if dependencies have changed).