lang: ast, interfaces, interpolate: Remove the legacy pos

This ports things to the new textarea. We need to plumb through things a
lot more, especially the string interpolation math to get the right
offsets everywhere, but that's coming.

lang/interfaces/ast.go

@@ -231,7 +231,7 @@ type Data struct {
 	// StrInterpolater is a function that needs to get passed in to run the
 	// string interpolation. This is passed in this way to avoid dependency
 	// cycles.
-	StrInterpolater func(string, *Pos, *Data) (Expr, error)
+	StrInterpolater func(string, *Textarea, *Data) (Expr, error)
 
 	// SourceFinder is a function that returns the contents of a source file
 	// when requested by filename. This data is used to annotate error