Inserting regex into literal regex expressions

(From the forums topic https://forums.swift.org/t/inserting-regex-into-literal-regex-expressions/81070.)

When a regex has been declared e.g. via a literal expression:

let greeting = /(hello|hi)/

Then it can be nice to use this in another regex (this does work today):

let greetingSomeone = Regex {
    greeting
    ChoiceOf{ " "; ", " }
    OneOrMore { .word }
}

It should be possible to insert the greeting regex directly into a literal regex (the syntax might have to be different):

let greetingSomeone = /\(greeting)( |, )\w+/

Start by reviewing the literal regex expression entry point and compare it with the existing RegexBuilder composition shown in the issue. Define the supported interpolation syntax and add coverage for embedding greeting in a literal; done when the example compiles and matches the intended greeting forms.