[New Feature]: Provide completion and related LS support for new regex support

[IMS94]

Description

Consider the following new addition to support regex:

 string:RegExp regex = re `[0-9a-${endChar}]`;

LS should support completion support etc here.

Related area

-> Editor

[dulajdilshan]

Related Github discussion: https://github.com/ballerina-platform/ballerina-lang/discussions/36805

[dulajdilshan]

Compiler FE changes: https://github.com/ballerina-platform/ballerina-lang/pull/37916

[dulajdilshan]

LS requirements can be found in the comments in RegExp KT

[pcnfernando]

Wondering if it is possible to support ReBaseQuantifier, ReSyntaxChar, and whatever we have a set of options. It shouldn't anyway be a completion since we can't be sure what the next node be. But maybe just suggestions when we do 'Ctrl + Space'?