Split up the "variants" CSS classes in text field into private mixins

[lynnmercier]

Part of our effort to split text field to reduce merge conflicts and generally improve the readability of text field code.

Should include:

• mdc-text-field--textarea
• mdc-text-field--full-width
• mdc-text-field--box
• mdc-text-field--leading-icon
• mdc-text-field--trailing-icon

[bonniezhou]

Resolved by #1862, #1885, #1899, #1942, #1964, #1988, #2050, #2051, #2053, #2054, #2055, #2057, #2068, #2071.