Co-authored-by: Valentin Touzeau <tree-sitter.github@valentintouzeau.fr>

This commit merges functions "elseBlock" and "elifBlock" in a single
function "alternativeBlock". Any call to either function is replaced by
a call to "alternativeBlock", which cover all alternatives (elif,
elifdef and else). The goal is twofold:

1) Correct the grammar to allow all kind of alternatives independently
   of the "if" directive. Before the fix, "#if" alternative was
   generated by elseBlock only, and thus disallow "#elifdef" as a
   follow-up.
2) As a side effect, simplify the parser.

Specifically for Neovim which contains code like:

```c
Dictionary nvim_eval_statusline(String str, Dict(eval_statusline) *opts, Arena *arena, Error *err)
  FUNC_API_SINCE(8) FUNC_API_FAST
{
  ...
}
```

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

Co-authored-by: Denny Sun <dennys@microsoft.com>

This resolves conflicts with declarations that can be interpreted as binary_expressions

* 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)

This also lets it bind to _any_ statement, which is necessary for some
c++ attributes that can attach to the statement in if, while, and
do/while statements.