Add cjs config support

[lewisl9029]

Changes

This PR adds support for using snowpack.config.cjs in addition to all of the existing config formats. This was a blocker for me when trying to use snowpack in a project that had "type": "module" specified.

Turns out all that was required was upgrading cosmiconfig to v7 and 1 line of code change.

One important consideration is that cosmiconfig v7 drops support for node 8, now requiring node 10+: https://github.com/davidtheclark/cosmiconfig/blob/master/CHANGELOG.md#700. Looks like snowpack already requires node version >=10.19.0 in engines, so I imagine this might not need to be considered a breaking change, but calling it out just in case we might want to save this for a major version for whatever reason.

I've also updated the appropriate section in the docs to include the new format. Let me know if there's any other changes you'd like to see.

Testing

I tested this manually by building locally and copy pasting the build to the project with snowpack.config.cjs config, and then manually bumping the version of cosmiconfig in that project. Seems to be working great.

Since this functionality is almost entirely handled by cosmiconfig, I didn't feel a new test here would add much value, but happy to add one if folks disagree.

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[FredKSchott]

This looks great! I have a question related to how we can document this: Would the config have worked as .js & ESM, if you had defined your config file as ESM?

// snowpack.config.js
// does this work?
export default {
  ...
}

[lewisl9029]

This looks great! I have a question related to how we can document this: Would the config have worked as .js & ESM, if you had defined your config file as ESM?

// snowpack.config.js
// does this work?
export default {
  ...
}

Here's the error I got when I tried this:

Error [ERR_REQUIRE_ESM]: Must use import to load ES Module: ...\snowpack.config.js
require() of ES modules is not supported.

I believe we'd need some special case handling for "type": "module" projects in cosmiconfig to use import instead of require for this to work, which might not be a trivial change (I haven't looked into their source code yet). In the mean time, this feels like a simple short term solution to unblock usage in "type": "module" projects.

[FredKSchott]

In the meantime, this feels like a simple short term solution to unblock usage in "type": "module" projects.

+1, thanks for the explanation and extra details