When `"npmCommand": ["bun"]` is configured in settings.json, pi fails to
start because it invokes `bun root -g`, which doesn't exist:
error: Failed to run bun root -g: error: Script not found "root"
Add a check for the Bun runtime using the logic already used elsewhere,
and build the relative path starting from Bun's bin directory.
c241c6d6d0