Add an efficient text stream write function (#14821)

This adds PR adds a couple foundational functions and classes to make
our TextBuffer more performant and allow us to improve our Unicode
correctness in the future, by getting rid of our dependence on
`OutputCellIterator`. In the future we can then replace the simple
UTF-16 code point iterator with a proper grapheme cluster iterator.

While my focus is technically on Unicode correctness, the ~4x VT
throughput increase in OpenConsole is pretty nice too.

This PR adds:
* A new, simpler ROW iterator (unused in this PR)
* Cursor movement functions (`NavigateToPrevious`, `NavigateToNext`)
  They're based on functions that align the cursor to the start/end
  of the _current_ cell, so such functions can be added as well.
* `ReplaceText` to write a raw string of text with the possibility to
  specify a right margin.
* `CopyRangeFrom` will allow us to make reflow much faster, as it's able
  to bulk-copy already measured strings without re-measuring them.

Related to #8000

## Validation Steps Performed
* enwik8.txt, zhwik8.txt, emoji-test.txt, all work with proper
  wide glyph reflow at the end of a row ✅
* This produces "a 咪" where only "a" has a white background:
  ```sh
  printf '\e7こん\e8\x1b[107ma\x1b[m\n'
  ```
* This produces "abん":
  ```sh
  stdbuf -o0 printf '\x1b7こん\x1b8a'; printf 'b\n'
  ```
* This produces "xy" at the end of the line:
  ```sh
  stdbuf -o0 printf '\e[999C\bこ\bx'; printf 'y\n'
  ```
* This produces red whitespace followed by "こ " in the default
  background color at the end of the line, and "ん" on the next line:
  ```sh
  printf '\e[41m\e[K\e[m\e[999C\e[2Dこん\n'
  ```

src/til/ut_til/RunLengthEncodingTests.cpp

@@ -190,7 +190,7 @@ class RunLengthEncodingTests
 
         // We're testing replace() elsewhere, but this is special:
         // This ensures that even if we're default constructed we can add data.
-        rle.replace(0, 0, { 1, 5 });
+        rle.replace(0, 0, rle_type{ 1, 5 });
         VERIFY_ARE_EQUAL(5u, rle.size());
         VERIFY_IS_FALSE(rle.empty());
     }