Rewrite HighlightedTextControl and remove HighlightedText (#19130)

`HighlightedTextControl` is a XAML user control that contains a text
block and takes vector of `HighlightedText`. `HighlightedText` uses
tuples `(string, boolean)`. Allocating an entire object to store a
string and an integer felt like a waste, especially when we were doing
it thousands of times for the command palette _and just to pass them
into another object that stores a string and a few more integers
(`Run`)._

The new `HighlightedTextControl` is a standard templated control, and
supports styling of both the inner text block and of the highlighted
runs.

It no longer takes a `HighlightedText`, but rather a standard string and
a set of runs (tuple `(int start, int end)`); these can be stored more
efficiently, and this change moves the construction of text and runs
directly into `HighlightedTextControl` itself as an implementation
detail rather than an API contract.

### XAML Properties

- `Text`: the string to highlight
- `HighlightedRuns`: a vector of `(start, end)` pairs, indicating which
  regions are intended to be highlighted. Can be empty (which indicates
  there is no highlight and that the entire string is styled normally.)
- `TextBlockStyle`: the `Style` applied to the inner text block;
  optional; allows consumers to change how both normal and highlighted
  text looks.
- `HighlightedRunStyle`: a `Style` applied only to the highlighted runs;
  optional; allows consumers to change how highlighted text looks. If
  left NULL, highlighted runs will be bold.

`HighlightedRunStyle` is a little bodgy. It only applies to `Run`
objects (which is fine, and XAML somewhat supports), but since `Run` is
not a `FrameworkElement`, it _doesn't actually have a `Style` member._
We need to crack open the style and apply it manually, entry by entry.
`FontWeight` is special because XAML is a special little flower.

.github/actions/spelling/expect/expect.txt

@@ -2,6 +2,7 @@ aaaaabbb
 aabbcc
 ABANDONFONT
 abbcc
+abcc
 abgr
 ABORTIFHUNG
 ACCESSTOKEN