Improve screen reader announcements for search box

[carlos-zamora]

Summary of the Pull Request

Improves the notification read out by a screen reader when a search is done in the terminal. This is done across several scenarios:

• previously, the results were only read occasionally. Moving the block out of if (results.SearchInvalidated) fixes that.
• previously, we read "Results found" or "No results found". Now, we read an accessible version of the written status message.

In order to maintain consistency with the written status message, _FormatText() now takes an isAccessible parameter to output a more accessible version of the text. Specifically...

• CurrentIndexTooHighStatus (aka ?) would not be read, so we replace it with TermControl_UnknownSearchResultIndex (aka unknown)
• TermControl_TooManySearchResults (aka 999+) would drop the + when read, so we replace it with TermControl_TooManySearchResults (aka over 999)
• TermControl_NumResults (aka {0}/{1}``) would be read as a fraction, so we replace it with TermControl_NumResultsAccessible(aka{0} of {1}`).

Validation Steps Performed

✅ Announcements are read out when interacting with search box (i.e. next, prev, regex toggle, etc.)
✅ "4 of 5" read out by screen reader (or something similar) when search is performed

Closes #19691

[carlos-zamora]

• TermControl_NumResults (aka {0}/{1}``) would be read as a fraction, so we replace it with TermControl_NumResultsAccessible(aka{0} of {1}`).

This is the main one I want double checked. Is that okay for localized scenarios?