Deprecated: Improve $.trim performance for strings with lots of whitespace

Deprecated: Improve $.trim performance for strings with lots of whitespace

Regex imp implementation takes `O(N^2)` time to trim the string when
multiple adjacent spaces were present.

The new expression require that the "whitespace run" starts from
a non-whitespace to avoid `O(N^2)` behavior when the engine would
try matching `\s+$` at each space position.

Closes gh-5068