Add precacheImportVariables option for faster dynamic import builds

[exogen]

Since we can't know what values Stylus variables will have at runtime until we actually render, we can't resolve dynamic imports (those with variables) ahead of time. That means encountering a dynamic import requires that we bail out, resolve the import with webpack, then try rendering again. For large import trees with many dynamic imports, the result can be many render attempts and a slow build.

This adds a new option, precacheImportVariables. If variables used in imports tend to be one of a few values, like Walmart's $tenant variable, then we can just guess ahead of time that we'll need to resolve certain paths before rendering. In many cases this can cut the build back down to a single render attempt.

Note that variables can still have any value at runtime regardless of precacheImportVariables, but supplying a precacheImportVariables value that results in a "cache hit" will make things faster.

To make this possible in a reasonable way, this switches import detection from a simple regular expression back to parsing each file and visiting the AST.

Results on one Walmart app:

• Before: 27 render attempts in 33 minutes
• After: 1 render attempt in 5 minutes

[dstevensio]

👍 I like what I see

[jeffrifwald]

LGTM