String methods added + code cleanup and fixes

Thanks to @CallumQuick we have some more string methods implemented!

• String.prototype.concat ( ...args )
• String.prototype.endsWith ( searchString [ , endPosition ] )
• String.prototype.includes ( searchString [ , position ] )
• String.prototype.indexOf ( searchString [ , position ] )
• String.prototype.lastIndexOf ( searchString [ , position ] )
• String.prototype.repeat ( count )
• String.prototype.slice ( start, end )
• String.prototype.startsWith ( searchString [ , position ] )

Thanks to @lennartbuit we have
trim/trimStart/trimEnd added to the string prototype

Bug fixes + better support for decimal digits from @topaxi

code tidyup from @Atul9