Add String.prototype.trimStart, trimLeft, trimEnd, trimRight

[jszczypk]

Feature request - add polyfill for String.prototype.trimStart, trimLeft, trimEnd, trimRight

trimStart and trimEnd are not supported in IE, Edge and FF<61, Chrome<66
trimLeft and trimRight are not supported in IE
https://caniuse.com/#search=trimStart
https://caniuse.com/#search=trimEnd

Polyfill for trimStart
https://github.com/FabioVergani/js-Polyfill_String-trimStart

[daytonlowell]

trimStart and trimEnd are non-standard. I would avoid those.

[jszczypk]

trimStart and trimEnd are in EcmaScript 2019
https://www.ecma-international.org/ecma-262/10.0/index.html#sec-string.prototype.trimend

trimLeft and trimRight are non-standard but popular.

[JakeChampion]

Added in #95