This changeset fixes an issue caused by #15991 where "chunked" escape
sequences would get corrupted. The fix is to simply not flush eagerly
anymore. I tried my best to keep the input lag reduction from #15991,
but unfortunately this isn't possible for console APIs.

Closes #16079

## Validation Steps Performed
* `type ascii.com` produces soft font ASCII characters ✅

(cherry picked from commit bdf2f6f)
Service-Card-Id: 91283205
Service-Version: 1.19

I've found that #16079 was never properly addressed (it still randomly
occurred after even after PR #16349), which later led to the issues
described in #16769 (nushell flickering due to too many flushes).

The crux of the fix is that this brings back the `_noFlushOnEnd` flag
that was removed in PR #15991. This is then combined with a change to
the cork API: An `uncork` on `VtEngine` now only flushes if `_Flush`
got called while it was corked in the first place.

`_noFlushOnEnd` prevents us from flushing in between two "unknown"
VT sequences (like soft fonts or FTCS) which prevents them from being
corrupted. The corking prevents the remaining cases of flushing too
often. Long-term, a proper fix would be to pass through VT unmodified.

Closes #16769

I've found that #16079 was never properly addressed (it still randomly
occurred after even after PR #16349), which later led to the issues
described in #16769 (nushell flickering due to too many flushes).

The crux of the fix is that this brings back the `_noFlushOnEnd` flag
that was removed in PR #15991. This is then combined with a change to
the cork API: An `uncork` on `VtEngine` now only flushes if `_Flush`
got called while it was corked in the first place.

`_noFlushOnEnd` prevents us from flushing in between two "unknown"
VT sequences (like soft fonts or FTCS) which prevents them from being
corrupted. The corking prevents the remaining cases of flushing too
often. Long-term, a proper fix would be to pass through VT unmodified.

Closes #16769

(cherry picked from commit 1ede023)
Service-Card-Id: 91965217
Service-Version: 1.20

I've found that #16079 was never properly addressed (it still randomly
occurred after even after PR #16349), which later led to the issues
described in #16769 (nushell flickering due to too many flushes).

The crux of the fix is that this brings back the `_noFlushOnEnd` flag
that was removed in PR #15991. This is then combined with a change to
the cork API: An `uncork` on `VtEngine` now only flushes if `_Flush`
got called while it was corked in the first place.

`_noFlushOnEnd` prevents us from flushing in between two "unknown"
VT sequences (like soft fonts or FTCS) which prevents them from being
corrupted. The corking prevents the remaining cases of flushing too
often. Long-term, a proper fix would be to pass through VT unmodified.

Closes #16769

(cherry picked from commit 1ede023)
Service-Card-Id: 91965216
Service-Version: 1.19