feat: expose public interface for locating Chrome installations

[knksmith57]

Add Launcher.getInstallations() public static method that returns an
array of paths to available Chrome binaries. Also, refactor
Launcher#launch() to use it.

Launcher.getInstallations() uses chrome-finder under the hood, so all
previous resolution logic is preserved:

• if CHROME_PATH env variable is set, it will be included
• if a Chrome Canary is detected, it will be included
• if a Chrome (stable) is detected, it will be included

Resolves #176

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[knksmith57]

@googlebot I signed it!

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[patrickhulce]

LGTM % nits

thanks @knksmith57!

[knksmith57]

OK looks like adding that additional assert worked (I didn't realize we codified the chrome dep in the travis config; I should have looked at that first 😅).

I think I've addressed all feedback --thanks again for the super fast review!

[patrickhulce]

yup the rest of our tests would fail if we didn't have chrome available so it's a safe bet :)

[patrickhulce]

looks great, thanks again @knksmith57! 🎉 💯

[paulirish]

great idea :D