Prepare to make shell integration the default

[Tyriar]

Shell integration issues (June): https://github.com/microsoft/vscode/issues?q=is%3Aopen+is%3Aissue+label%3Aterminal-shell-integration+milestone%3A%22June+2022%22
Shell integration issues (all): https://github.com/microsoft/vscode/issues?q=is%3Aopen+is%3Aissue+label%3Aterminal-shell-integration

We probably don't want to make it the default in this milestone still but we can focus some effort on the parts that must be done to enable it by default. Such as:

• Improved testing - we had some flakiness issues with smoke tests but I believe they may be fixed after Make sure PS1/2 aren't re-wrapped in bash shell integration #151232, the matrix of shells we need to test is quite large and not realistic to test everything on every change. We should look into how we can test these things without worrying about flakiness.
  • Terminal shell integration smoke tests flaky #150478
  • Enable command decoration smoke tests on Windows #148082
  • Investigate alternative approaches to shell integration testing #152658
• Fixing common reliability issues on Windows
  • Single line pwsh prompt isn't handled as well as multi-line #151929
  • Windows/pwsh shell integration doesn't work when VS Code's install path has a space #151323
• Prevent multiple decorations from appears on top of one another
  • Harden shell integration handlers in renderer to handle multiple start sequences (A, B) properly #151228
  • Placeholder decoration overlaid with success/error decoration #144597
• Look into injection failure detection and recovery
  • Detect shells that fail to launch when shell integration injection fails #151934
• Test more prompts and cases where we should be disabling shell integration due to incompatibility - I'm still not confident we won't break a bunch of shells when it goes live which could potentially brick the terminal
  • Bash Shell Integration: Please co-exist with other trap DEBUG handlers (ie: leverage bash-preexec) #150241
  • Shell Integration indicator always empty circle when PROMPT_COMMAND is two functions #151757
  • ...
• Support manual install for complex prompts or sub-shells
  • Allow running shell integration scripts inside an rc file #151933
  • Shell integration: Explore how to handle sub-shells #141343
  • Create a user path for setting up shell integration manually vscode-docs#5219

Not a must but nice to have:

• Support standard shell integration sequences as a fallback Support shell integration in oh-my-zsh and powerlevel10k #146587

[Tyriar]

This is basically done, in a much better state now. Still have the open tracking issues for remaining tasks.