Granular scss rebuild

[aleclarson]

For each .scss or .sass module:

• Track declared variables/mixins
• Track external variables/mixins being used

When a module is updated, insert external variables at the top of the module before transpiling.

This strategy lets us avoid re-transpiling the entire bundle when a .scss or .sass module changes.

Note: We won't support cross-file @extend for now.

[aleclarson]

This is more effort than it's worth, I think.

I completed the reference/declaration parser and got cold feet after digging into the complexity of resolving and tracking dependencies between modules.

May revisit in the (not nearby) future.

[aleclarson]

This would be easier to implement if only direct parents have access to the variables, mixins, and functions of a module.