add advanced mount configuration

[FredKSchott]

Mentioned a few times in discord, requested by @rauschma in #875 (zero build/resolve mode) & mentioned as useful by the Svelte team.

Changes

• Adds ability to turn on/off file building & import resolution on a per-mount basis.
• This is advanced usage and not needed for most, since only non-web languages (jsx, ts, tsx, etc) build by default and Snowpack rarely needs to resolve JS imports that would already run directly in the browser.
• But, if you know what you're doing, this config can give more explicit guarantees and speed up your build a bit.

Testing

• "mount" test added.

Docs

• Better "mount" config docs added.

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