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.