[templates]: Add support for yarn

[ThorstenHans]

This PR adds support for yarn as an alternative package manager to all templates in the repository.

[karthik2804]

@ThorstenHans the above suggested fix needs to be fixed for all the templates.

[ThorstenHans]

already pulled the branch... will update in a sec

[radu-matei]

Anything else blocking this PR?

[ThorstenHans]

IMO, we wanted to time this PR with recent refactorings. Adding @karthik2804

[ThorstenHans]

I've rebased this PR and incorporated all the recent changes. cc @karthik2804

[ThorstenHans]

We're currently facing an issue when using yarn as package manager. Although, building the Spin component does not result in an error, the resulting component won't run at all.

The wasm component does not provide necessary exports. It looks like knitwit does not load KNITWIT_SOURCE from the corresponding environment variable. Instead, it's only pulling from npm_config_knitwit_source or falling back to defaults... cc @karthik2804

[karthik2804]

The issue seems to be how npm and yarn treat case sensitivity differently for env vars in .npmrc. Marking as draft as in the next major release, we will not need to depend on the env vars for build and add yarn as a part of it.