Helper text attribute for input components

[fredvisser]

🙋 Feature Request

😯 Problem to Solve

The control text pattern, documents the use of Helper text that:

• Optional, provides information to help users enter values correctly.
  • Include in Helper text if most users need it.
  • Include in Toggle tip if only a few users need it.
• Do not use Placeholder text for information users may need while entering data.
• Prefer sentence fragments, but use full sentences as necessary to ensure clarity for complex information.

…but we do not current support helper text as a first-class attribute for nimble input controls.

💁 Proposed Solution

• Add a helper-text and helper-visible attribute to each of the number-field, text-field, select, and combobox components.
• Consider adding to the radio-group, and switch components.
• If both helper and error text are configured to be shown, error text would win

📋 Tasks

[jattasNI]

Before building this we should write a short HLD to resolve open questions like what the exact API should be and how this would appear in the accessibility tree.

[rajsite]

Related, #2830 is introducing the concept of a single severity-text property (as of writing) and does not add different "warning-text", "information-text", etc. to compliment "error-text".

Maybe it's good to have the default be separate. Analogous to placeholder (always around) vs value (based on current state).

API direction for controls that only express default + error severity:

• error-text
• helper-text (if set, visible unless error-visible is set and non-empty error-text)
• error-visible

API direction for controls that express all severities:

• severity-text (used for non-default severity)
• helper-text (used for default severity)
• severity (determines which text is shown)