Add support for IES

[schveiguy]

Interpolation Expression Sequences were added in 2.108.

The documentation is here: https://dlang.org/spec/istring.html

DCD and other tools (D-Scanner, etc) should properly deal with these, and provide completion for while you are inside an interpolated piece of the literal.

[WebFreak001]

perhaps you can provide input to dlang-community/libdparse#510 to get the PR in quicker