No longer use `str` in speech sequences

[feerrenrut]

Is your feature request related to a problem? Please describe.

Improving semantics for internal speech processing is held back by maintaining add-on compatibility.

Describe the solution you'd like

• Make str and invalid speech sequence item.
• All text should be wrapped to give semantics. See UserInterface and Symbol commands in #12710
• A common interface for all text content (but with more semantics than str) allows addons to consume this, while giving NVDA core developers flexibility to extend and refine the semantics used.

EG: Once addons all use:

for command in sequence if isinstance(command, TextContent):
    command.text = f"Add more info: {command.text}"

We can gradually create subclasses of TextContent with more specific semantics:

• Symbol
• UserInterface
• Content
• Context (https://github.com/nvaccess/nvda/pull/12710#issuecomment-1031277294)

Then we can offer features like UserInterface uses a different voice from content.

Describe alternatives you've considered

None

Additional context

Very relevant to this is issue #12778 "Split speech processing commands and commands for synth"