schema: Add string interpolation UAST type

[dennwc]

I identified another low-hanging fruit in terms of Semantic UAST types: string interpolation.

The node is similar if most drivers I've seen, and the semantic is pretty well-understood. Essentially all nodes of this kind follow the following structure:

type StringInterpolation struct{
  Parts []Any
}

Each part can be a String, Identifier or any other expression that yields a value. The effect of this operation is to convert all arguments to String and join them into a single one.

Current list of discovered interpolation nodes:

• bash:string
• csharp:InterpolatedStringExpression
• javascript:TemplateLiteral
• php:Scalar_Encapsed
• python:JoinedStr
• ruby:dstr

If we decide to go full-in on Semantic, there some other candidates:

• Go: fmt.Sprintf, strings.Join(, "")
• Java: String.format
  But for now, I propose to only touch the unique AST nodes, not function calls.

In terms of functionality, this will allow to better detect SQL injections and similar bugs.

[juanjux]

Python has two variants:

"some string {w} named interpolation".format(w='with')
# or
"some string {0} positional {1}".format('with', 'interpolation')
# or
"some string {} implicit positional {}".format('with', 'interpolation')
# this one is the joinedstr:
f"f-string {w} some interpolated variable"

Joined strings are the second ones. I've just noticed that the first one doesn't have an integration test so I'll make a PR for it.

The format ones get a pretty complicated AST: Expression->Call where the args are the {}'s for the second and third case or keywords for the third and the string itself is inside func.QualifiedIdentifier.identifiers[0].