Create a set of classes for components of regex literals,
separate from those of string literals. This allows us to special-case
components of free-spacing regexes (ones with the /x flag) to not have a
`getValueText()`.
This in turn is useful because our regex parser can't handle free-spacing
regexes, so excluding them ensures that we don't generate erroneous
ReDoS alerts.
When calculating `StringlikeLiteral.getValueText`, include results from
interpolations where we can determine their string value. For example:
b = "b" # local variable
D = "d" # constant
"a#{b}c" # getValueText() = "abc"
"a#{b}c{D}" # getValueText() = "abcd"
/#a#{b}c{D}/ # getValueText() = "abcd"
