Bundle extension as ESM

[bwateratmsft]

Code loading time jumped a bit in 2.4.0, which isn't super surprising because esbuild is putting everything into a single file instead of code splitting as Webpack used to do.

We should try out an ESM extension to get code splitting back.

[bwateratmsft]

Currently blocked by:

• eng: ESM extension fails to load due to duplicate import of createRequire vscode-azuretools#2181 (update: DONE!)