Add `quarto convert` command

[jooyoungseo]

quarto convert command is currently missing in VSCode extension, which would be great to add.

[jjallaire]

Thanks for this, yes , we would definitely like to add this (note that the currently philosophy of the vscode extension is that it complements rather than replaces the use of the terminal -- e.g. things like publish, installing extensions, etc. all still require the terminal).