Warn about `TARGET = this`

[amtep]

Tiger should warn about this common error pattern:

• you pass FOO=this or FOO=prev to a scripted effect, trigger, or modifier
• you use $FOO$ in the scripted script but not at its top scope level

In theory you could make this be correct, but it's a very common error. The this or prev will be evaluated in a different context than expected.