repl: backslash bug fix

1st commit:

When some string is copied and pasted in REPL, if it has a newline (\n) in it, then it is considered as end of line and REPL throws a SyntaxError, because a line cannot end with a unclosed string literal. This behavior is because of the fact that readline module breaks at all the \ns in the input string and treats them as a seperate line. So whenever it encounters a new line character it will emit a line event.

This commit basically reverts 81ea52aa, which was an attempt to improve the way string literals were handled by REPL.

Fixes: #2749 (closed)

2nd commit

As it is, the trimWhitespace function will not remove the white sapce characters at the end of the string, as the greedy capturing group .+ captures everything till end of the string, leaving \s* with nothing.

This patch replaces the buggy function with the built-in, String prototype's trim function.

Note: The second commit is necessary because, the revert brings back the old buggy trimming function. So, I though we could fix it now itself.

cc @silverwind @Fishrock123