With us adding a .appinstaller distribution of Canary, the Store
services update checker has beome insufficient to determine whether
there are package updates.

App Installer supports us checking for updates by using PackageManager
and the Package interfaces.

We'll use those instead of the Store services interface, and bail out
early if the App Installer gives us an answer.

A carriage return (enter key) will increase the _distanceEnd by up to
viewport-width many columns, since it increases the Y distance between
the start and end by 1 (it's a newline after all).
This will make _flushBuffer() think that the new _buffer is way longer
than the old one and so _erase() ends up not erasing the tail end of
the prompt, even if the new prompt is actually shorter.

This commit fixes the issue by separating the newline printing
out from the regular text printing loops.

## Validation Steps Performed
* Run cmd.exe
* Write "echo hello" and press Enter
* Write "foobar foo bar" (don't press Enter)
* Press F7, select "echo hello" and press Enter
* Previous prompt says "echo hello" ✅

Subjectively speaking, this commit makes 3 improvements:
* Most importantly, it now would work with arbitrary Unicode text.
  (No more `IsGlyphFullWidth` or DBCS handling during reflow.)
* Due to the simpler implementation it hopefully makes review of
  future changes and maintenance simpler. (~3x less LOC.)
* It improves perf. by 1-2 orders of magnitude.
  (At 120x9001 with a full buffer I get 60ms -> 2ms.)

Unfortunately, I'm not confident that the new code replicates the old
code exactly, because I failed to understand it. During development
I simply tried to match its behavior with what I think reflow should do.

Closes #797
Closes #3088
Closes #4968
Closes #6546
Closes #6901
Closes #15964
Closes MSFT:19446208

Related to #5800 and #8000

## Validation Steps Performed
* Unit tests ✅
* Feature tests ✅
* Reflow with a scrollback ✅
* Reflowing the cursor cell causes a forced line-wrap ✅
  (Even at the end of the buffer. ✅)
* `color 8f` and reflowing retains the background color ✅
* Enter alt buffer, Resize window, Exit alt buffer ✅