Add methods to remove specified leading/trailing parts of a string

[Reprevise]

Dart should have methods that are able to remove specified leading and trailing parts of a string. I propose using strip and it's associates, stripLeft and stripRight to keep in line with trim.

"@bar".stripLeft('@'); // "bar"
"bar@".stripRight('@'); // "bar"
"@bar@".strip('@'); // "bar"

"@@bar".stripLeft('@'); // "bar"
"@@bar".stripLeft('@@'); // "bar"
"@@bar".stripLeft('@@@'); // "@@bar"
"@@@bar".stripLeft('@@'); // "@bar"

The signature for the methods would be:

String strip(String pattern) {} // Only String, see lrhn's comment about matching backwards with RegEx

String stripLeft(Pattern pattern) {}

String stripRight(String string) {} // Only String, see lrhn's comment about matching backwards with RegEx

[FMorschel]

CC @lrhn

[lrhn]

Could be useful.

For trimLeft we could take a Pattern for what to remove, but we can't do that for trimRight, since patterns can only match forward.
(Trying to match backwards using the Pattern API will be inefficient and inconsistent.)

That means that there is no user argument that would behave the same as the default trimRight.

Also, I want the trim functions to be efficient, so I'd rather add new functions, than have to branch on the arguments.

I've been working on something like removePrefix which removes a known prefix string from a string, with a variant that does nothing if the prefix isn't there.
Giving such a function a parameter that allows it to remove multiples would be a more consistent API, than adding it to trim which works in any whitespace.

The word strip also works.

[Reprevise]

I've updated the issue to remove any mention of altering trim and to strictly use strip (w/ stripLeft and stripRight).