Adjust string rules to flatten parser tree to one node per string lit…

[nordlow]

…eral instance and enforce suffix _literal for all string literal rules.

This makes D's grammar harmonize more with tree-sitter-c and tree-sitter-cpp.

Will regenerate parser if and when this gets approved.

It might be more suitable to just name regular_string_literal for string_literal as these exist in tree-sitter-c and tree-sitter-cpp.

Note that _any_string_literal corresponds to tree-sitter-cpp's _string.

[gdamore]

So I'm not sure about the motivation for matching other tree-sitter grammars. D's grammar for strings is quite a lot more complex than either C or C++. Conversely, these rules are now coded into quite a number of queries across a swathe of editors, so changing this will break things for them.

I'm not saying I won't do it, but I'm not convinced the benefit here is worth it.

The fact that these are all classes of string_literal actually helps some of the queries for simpler editors, because you don't have to know all the varieties of string literal to get a reasonable highlighting.