build(workspace): set 'transpileOnly' mode for ts-loader with 'BUILD_TRANSPILE_ONLY' env var

[ascariandrea]

Summary

The CI is not checking the tk:ext is compiling properly, cause previously our webpack configuration only transpiled the source code. So I'm adding a step for yarn tk:ext tsc -b and check the extension compiles properly and set BUILD_TRANSPILE_ONLY env var to control type checking in webpack

Changelog

[build:workspace] build(workspace): set 'transpileOnly' mode for ts-loader with 'BUILD_TRANSPILE_ONLY' env var

Test Plan

# comment the `logoUrl` property from `tk:ext` popup
yarn tk:ext watch # no errors
yarn tk:ext build # ts error
# you can repeat the test for `yt:ext` too

[rekoke]

no errors, all good!