Bug: mdc-textfield's root index.js file does not export the foundations or default implementations of its sub-components

[treybrisbane]

Currently (as of v0.26.0), mdc-textfield's root index.js file does not export any of the following:

• MDCTextFieldBottomLine
• MDCTextFieldBottomLineFoundation
• MDCTextFieldHelperText
• MDCTextFieldHelperTextFoundation

Imports for these symbols were added to the root index.js as part of #1684, however they were not subsequently exported.

By not exporting these symbols from the root index.js, we provide no way for consumers to access them from dist/mdc.textfield.js bundle. Consumers are instead forced to access them via their ES6 modules, which can be problematic for some (see #929).

[tyronedougherty]

+1 on this