This is GNU extension and GNU extensions seems to be prefixed with
'gnu_' in other places of source code.

Now allows `<string> <identifier> ...` forms of concatenations.

We do ensure at least one string is present though, otherwise conflicts arise

* feat: allow identifiers in string concatenations

Resolves errors in strings of the form:

   ("failed in line %" PRIdLINENR)

---------

Co-authored-by: Lewis Russell <lewis6991@gmail.com>

According to cppreference, identifiers are `(XID_Start | '_')
XID_Continue*`, which is the case as of C++23 and C2x. I have confirmed
this myself with the drafts of C++23 and C2x.

https://en.cppreference.com/w/cpp/language/identifiers

Clang indeed implements identifiers as `(XID_Start | '_') XID_Continue*`
in C++ mode and C2x mode, with a slight extension to the character set
to include some extra math characters:
231992d9b8/clang/lib/Lex/Lexer.cpp (L1517-L1530)

* Fix error with ++L (consequence of support for L strings)

* Update to tree-sitter 0.15.5

Fixes atom/language-c#323

Fixes atom/language-c#312

Fixes tree-sitter/tree-sitter-cpp#24
Fixes tree-sitter/tree-sitter-cpp#34

Fixes tree-sitter/tree-sitter-cpp#26