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.

This fixes a slight regression when parsing a preproc function definition using division operators

* feat: allow identifiers in string concatenations

Resolves errors in strings of the form:

   ("failed in line %" PRIdLINENR)

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

* #include can be followed by a macro name or a macro call

* In preproc_include, use preproc_call_expression

* Macro definition can contain continuation line in the argument list

* Reset grammar since line continuations are handled globally

Preserve the test.

* Parse expressions in preprocessor #if statement

* Make preproc_expression look like a normal expression

* Hide the wrapper rules
* Alias the rulres to match their corresponding expression rule
* Handle line continuations as extra tokens

Co-Authored-By: Thomas Johansen <thomasjo@gmail.com>

Signed-off-by: Nathan Sobo <nathan@github.com>

Signed-off-by: Nathan Sobo <nathan@github.com>

Signed-off-by: Nathan Sobo <nathan@github.com>

Doing this required treating the newline that follows these conditions as significant
in order to avoid conflicts, but this caused problems for error recovery.

Now there is never a need to mark a repeat rule as optional