enable typechecking for the babel-plugin directory

[samouri]

First step towards #35573

[amp-owners-bot]

Hey @erwinmombay, @jridgewell! These files were changed:

build-system/babel-plugins/babel-plugin-amp-mode-transformer/index.js
build-system/babel-plugins/babel-plugin-const-transformer/index.js
build-system/babel-plugins/babel-plugin-imported-helpers/index.js
build-system/babel-plugins/babel-plugin-transform-amp-asserts/index.js
build-system/babel-plugins/babel-plugin-transform-amp-extension-call/index.js
build-system/babel-plugins/babel-plugin-transform-annotation-to-extern/index.js
build-system/babel-plugins/babel-plugin-transform-default-assignment/index.js
build-system/babel-plugins/babel-plugin-transform-dev-methods/index.js
build-system/babel-plugins/babel-plugin-transform-fix-leading-comments/index.js
build-system/babel-plugins/babel-plugin-transform-function-declarations/index.js
build-system/babel-plugins/babel-plugin-transform-html-template/index.js
build-system/babel-plugins/babel-plugin-transform-inline-isenumvalue/index.js
+13 more

Hey @rileyajones! These files were changed:

build-system/tsconfig.json