Adding support for alignas type qualifier

[touzeauv]

This commit adds support for alignas type qualifier, as asked in issue #206.

I do not fully understand the current state of the tree-sitter-c grammar, I might have done some mistake.
In particular, please double check the following point:

• I added "alignas" as a type_qualifier whereas cppreference considers it is a "type specifier". I am not sure I understand the distinction between "specifier" and "qualifier" but type_qualifier seemed more appropriate to me.
• I chose to make alignas_qualifier nodes visible in the tree. I don't have an intuition about when to hide, show or rename nodes in the tree, so I might be wrong here.

The commit includes a small modification of the test corpus to avoid future regressions.

PS: Thanks for your work maintaining tree-sitter

[amaanq]

Ah interesting - I added support for this in CPP, but I think your change makes sense to have in C and have C++ inherit it. There's one lingering question though, should the second choice be type_descriptor, or would _type_specifier suffice? If there can be qualifiers in the type inside the alignas type qualifier then this is good, but just want to double check

[touzeauv]

I think type_descriptor is required for the second choice. cppreference the argument of alignas is a type-id, which might include qualifiers I believe.
Testing gcc and clang, both accept things such as alignas(volatile const int) char c;

Regarding C++, it seems the language only allows alignas, but not _Alignas.
clang++ is permissive and will parse it without complaining, but g++ complains that _Alignas is not declared.