feat(textfield): Improved Helper Text and Character Counter

[trimox]

• Remove MdcHelperText component
• Remove MdcCharacterCounter component
• Renamed property characterCounter to charCounter
• Add helper: string to MdcTextField
  • Helper text to display below the input when focused.
• Add helperPersistent: boolean to MdcTextField
  • Always show the helper text despite focus.
• Add validationMessage: string to MdcTextField
  • Message to show in the error color when the textfield is invalid. (Helper text will not be visible)
• Decouple MdcTextField and MdcTextarea documentation
• [x] Update documentation
• [x] Update examples
• [x] Update unit tests
• [x] Include breaking change notes

BREAKING CHANGES:

• Renamed property characterCounter to charCounter
• Removed MdcHelperText component, use helper: string instead
• Removed MdcCharacterCounter component
• Validation messages are set using validationMessage: string. See examples
• Helper text is set using helper: string. See examples
• Persistent helper text is enabled using helperPersistent: boolean. See examples

closes #2232 closes #2228 closes #2229 closes #2230