add `FormattedText` component

Defined as something like

class FormattedText(pydantic.BaseModel, extra='forbid'):
    text: str
    text_format: typing.Literal['bold', 'italic', 'underline', 'strikethrough'] | None = pydantic.Field(None, serialization_alias='textFormat')
    # TODO, use pydantic-extra-types Color?
    text_color: str | None = pydantic.Field(None, serialization_alias='textColor')
    background_color: str | None = pydantic.Field(None, serialization_alias='backgroundColor')
    type: typing.Literal['FormattedText'] = 'FormattedText'

The items in components like Button and Link become list[str | FormattedText | Link]. We might want too provide a few more components specifically for typography, like Bullets and Numbers

Two reasons to do this over relying on Markdown:

1. Markdown is a relatively heave lazy module, that it would be nice not to have to load
2. Markdown by it's definition is terrible for templating or user defined values - it's virtually impossible to escape. Providing these types would allow text to be defined which included uses values safely