Also unify all imports of `JSContext`.
All call-sides were automatically migrated using the following regex
replacement (previous regexes had a `;` at the end):
Before:
```
Promise::new\([ \n]*(.+),[ \n]+CanGc::from_cx\(cx\),?[ \n]*\)
.reject_error\([ \n]*(.+),[ \n]+CanGc::from_cx\(cx\),?[ \n]*\)
.resolve_native\([ \n]*(.+),[ \n]+CanGc::from_cx\(cx\),?[ \n]*\)
.reject_native\([ \n]*(.+),[ \n]+CanGc::from_cx\(cx\),?[ \n]*\)
```
After:
```
Promise::new2(cx, $1)
.reject_error_with_cx(cx, $1)
.resolve_native_with_cx(cx, $1)
.reject_native_with_cx(cx, $1)
```
Part of servo#40600
Testing: it compiles
Signed-off-by: Tim van der Lippe <tvanderlippe@gmail.com>