Support TypeScript

[simonihmig]

When RFC800 has landed, especially the --typescript flag for ember addon (see https://emberjs.github.io/rfcs/0800-ts-adoption-plan.html#cli-integration), we should support that here as well. Especially as we need to compile TypeScript in rollup, so support in addons is not as trivial as for v1 addons (ember install ember-cli-typescript).

Some prior art:

• https://github.com/NullVoxPopuli/ember-addon-v2-typescript-demo/blob/main/my-addon/config/rollup.config.js
• https://github.com/NullVoxPopuli/ember-resources/blob/main/ember-resources/config/rollup.config.js
• https://github.com/simonihmig/tracked-redux/blob/master/packages/tracked-redux/rollup.config.js

[simonihmig]

There has been some discussion about problems transpiling decorators:

• Add typecsript sample rollup config + exploration of different plugins' side-effects embroider#1099
• Discord thread at https://discord.com/channels/480462759797063690/568935504288940056/978501243880218714

We might want to only let Babel transpile code, including decorators, and let tsc only emit types. Like in https://github.com/evoactivity/ember-fast-marquee/pull/1/files

[NullVoxPopuli]

all problems that I'm aware are resolved atm.

the last thing is getting strict about extensions in the rollup plugin configs

[simonihmig]

For the record, we don't actually need to wait for official --typescript support in ember addon, as it turned out we can support additional flags even when the base command does not declare them.

So I would suggest we add this asap here once all remaining questions on embroider-build/embroider#1099 are resolved and the PR is merged. This can then serve as the "blueprint" (pun intended) for the implementation here...