ActivityBarService#showActivity returns a disposable, remove clearActivity, make message stack properly

Fixing sorting of directory and filenames with numbers (fixes #17495)

This reverts commit 7eeae68c.

This brings in namely:

- cursorBlink and scrollback refresh immediately after setOption call

This is the first build to feature the upgrade to winpty@0.4.1 in addition to
the conversion to TypeScript and general code clean up. The most notable
change is that Git Bash no longer goes out of sync and likely a bunch of other
issues in #14613. Certain applications within WSL do not function correctly
with this build.

I tested this on Windows and Linux.

Part of #13625

Fixes #18330

Left over from recent refactor, It's now handled in IShellLaunchConfig.

Setting Powershell as default terminal for Windows 10+ (fixes #16838)

* Simplify markdown paragraph logic

* Fix for alt headers

Also support relative paths

Fixes #13138

Easter egg: this commit was done via the VSCode PS shell!