docs: add tip for conditional shims using TERM_PROGRAM

[htunnicliff]

I added a tip explaining how to use the TERM_PROGRAM environment variable to conditionally use shim-based activation.

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For background, I use zsh and have my .zprofile configured to activate mise like so:

eval "$($HOME/.local/bin/mise activate zsh)"

However, as these docs already mention, IDEs such as VSCode don't always function as expected unless mise is activated with --shims.

To get the best of both worlds, mise activation can be conditionally modified if terminal detection is used, which is what I added to the docs:

# ~/.zprofile
if [[ "$TERM_PROGRAM" == "vscode" ]]; then
  eval "$($HOME/.local/bin/mise activate zsh --shims)"
elif; then
  eval "$($HOME/.local/bin/mise activate zsh)"
fi