feat(input): add helper and error text

[liamdebeasi]

Pull request checklist

Please check if your PR fulfills the following requirements:

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)
  • Some docs updates need to be made in the ionic-docs repo, in a separate PR. See the contributing guide for details.
• Build (npm run build) was run locally and any changes were pushed
• Lint (npm run lint) has passed locally and any fixes were made for failures

Pull request type

Please check the type of change your PR introduces:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

What is the current behavior?

Issue URL: N/A

When using ion-item developers can provide optional helper and error text to assist with data entry.

What is the new behavior?

• Adds functionality + styling for helperText
• Adds functionality + styling for errorText
• Adds styling for the "bottom content" (the area below the input/label that will house the helper text, error text, and counter)
• Adds the 'legacy-input' class to make it easier to detangle styles that should only apply to the legacy input.
• The error and helper slots on ion-item have been deprecated.

Note: counter logic will be added in a separate PR.

Does this introduce a breaking change?

• Yes
• No

Other information