add a new lint for preferring throw over return ThrowCompletion(...)

[michaelficarra]

Disclaimer: I used AI in the process of creating this PR.

[bakkot]

You don't need walkExpr for this; you only really need that when descending recursively into expressions. Here return ThrowCompletion(...) can only appear at the top level (i.e., not within another expression).

Instead you can just walk over stepSeq directly, looking for text nodes ending in return followed by call ThrowCompletion nodes instead of reading the raw source, which is also better because parsed source accounts for <ins> and <del> and etc.