Add scope types "trailing" / "leading"

[pokey]

Main use case being "chuck trailing" / "chuck leading" / "clear trailing" / "clear leading".

Need this in order to merge in the "move air after bat" branch

• Also look for tests that have a single target with non-null position to see if they should change to using trailing / leading instead. Esp tests introduced in Updated leading and trailing delimiters for before and after positions #340

[AndrewDant]

I'm not clear on what these scope types are meant to represent. Leading/trailing whitespace?