feat(list): add feature targeting for styles

[crisbeto]

Sets up feature targeting for mdc-list.

Here's the before and after output:
before.css.txt
after.css.txt

Some notes:

• There are a couple of differences where some rules swapped places which don't seem to be an issue, based on a screenshot diff of the demo page before and after.
• There are some differences where I found two rules with identical styles that could be combined into one.
• There are a bunch of tiny rules (1-2 lines of styles each) that could technically be put under one feature target. I decided not to, because the mixin is getting pretty large and it would make things harder to follow.

[crisbeto]

@kfranqueiro I've addressed the feedback. Note that I didn't move the styles from mdc-typography-overflow-ellipsis, mdc-typography-baseline-top and mdc-typography-baseline-bottom under the typography target, because the styles themselves seem more structural (margin, overflow, vertical-align etc.).

[kfranqueiro]

This line is in the wrong place. This belongs inside .mdc-list--dense & { below.

[crisbeto]

@kfranqueiro I've addressed the latest set of feedback.

[kfranqueiro]

No screenshot diffs in list or menu: https://storage.googleapis.com/mdc-web-screenshot-tests/kfranqueiro/2019/02/04/15_49_03_549/report/report.html?utm_source=cli&utm_medium=test_results