What does this PR do

Visual studio code IDE automatically updates extensions only when it is opened. It forces then to reload its windows, which is a (small but existing) annoyance.
I use Topgrade regularly with great success but observed it does not yet handle these extensions. I consequently made this PR to add this update.

Vscode extensions cli update is not trivial because no universal upgrade command exists ( a coresponding issue has been opened but not implemented). The steps I use are:

• get list of installed extensions with code --list-extensions (obtain a line-return separated list of installed extensions)
• construct the upgrade command and execute it
  • code --force --install-extension [ext0] --install-extension [ext1] ... --install-extension [extN]

I tested myself on Ubuntu 20.04 (including the case when no extension is installed) and on Windows 10.

Standards checklist:

• The PR title is descriptive.
• I have read CONTRIBUTING.md
• The code compiles (cargo build)
• The code passes rustfmt (cargo fmt)
• The code passes clippy (cargo clippy)
• The code passes tests (cargo test)
• Optional: I have tested the code myself
  • I also tested that Topgrade skips the step where needed

If you developed a feature or a bug fix for someone else and you do not have the
means to test it, please tag this person here.