Add support for PostCSS dependency messages

[bradlc]

Changes

This pull request adds support to @snowpack/plugin-postcss for dependency messages created by PostCSS plugins. These messages are used to signify to runners that the PostCSS compilation depends on files or directories other than the input file. The CSS should be rebuilt when these change.

Relevant documentation: https://github.com/postcss/postcss/blob/main/docs/guidelines/runner.md#3-dependencies

Testing

I'm really not sure how to add tests for this. Would appreciate any input on that!

Docs

No docs added as this is not a user-facing change. The messages are created by PostCSS plugins and handled internally by @snowpack/plugin-postcss

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[drwpow]

Love this! This is a great improvement to the plugin.

Re: tests, behavior like this is notoriously-difficult. As long as you’ve manually tested, this, then that should be enough. Generally-speaking, dev server behavior doesn’t get touched much so this probably won’t regress.

[bradlc]

Love this! This is a great improvement to the plugin.

Re: tests, behavior like this is notoriously-difficult. As long as you’ve manually tested, this, then that should be enough. Generally-speaking, dev server behavior doesn’t get touched much so this probably won’t regress.

Yep, we are working on a version of Tailwind JIT that uses the new dir-dependency messages and it works great! 🙌

[drwpow]

Merged, and going to release a new minor version with this improvement. Look for it in a few min!

[bradlc]

Thanks @drwpow!