[6.x] Add Info fieldtype

[joshuablum]

What

Adds a non-data Info fieldtype for displaying visual guidance in Control Panel publish forms. It supports sanitized Markdown, built-in icon selection, and five states:

• Notice (gray)
• Tip (blue)
• Warning (yellow)
• Important Warning (red)
• Success (green)

This PR is stacked on #15367, which provides the blue tip Alert variant.

Testing

• Added PHP tests for fieldtype behavior and configuration.
• Added frontend tests for every state, Markdown links/lists and sanitization, and custom icons.
• Confirmed the PHP fieldtype suite and integrated Alert/Info component tests pass.

Closes statamic/ideas#1359.