Automatically declare external variables for string sources

For string-source TextScripts the external variables passed to the run command should be automatically declared.

Currently, using external variables with string-source scripts is a bit difficult:

> run -s "## extern str ##@str;" --vars str="Test"
Test

Goal of this issue is to be able to write:

> run -s "@str;" --vars str="Test"
Test

(without the implicit-external-vars option).