Provide simpler route for activating shell integration

[Tyriar]

Fixes #153921

# pwsh
if ($env:TERM_PROGRAM -eq "vscode") { . "$(code --shell-integration pwsh)" }

# bash
[[ "$TERM_PROGRAM" == "vscode" ]] && . "$(code --shell-integration bash)"

# zsh
[[ "$TERM_PROGRAM" == "vscode" ]] && . "$(code --shell-integration zsh)"

[felipecrs]

Another approach would be to output the whole script when calling such command instead of script path. Like Homebrew's shellenv:

eval "$(brew shellenv)"

Or other tools which generates completion scripts, like:

kubectl completion zsh

[Tyriar]

@felipecrs I think since we'll say to use the if term_program is vscode, the eval should work the same as sourcing the file

As for completions, shell integration doesn't need completions to work, but we have some early prototypes for how to tackle that problem starting with pwsh #154662

[meganrogge]

Did not test but lgtm