Fix C++ synatx highlighting error in macros with extern "C"

[sryze]

Description of the Change

Update the C++ grammar to fix syntax highlighting of extern "C" inside macros.

Example code:

#ifdef __cplusplus
    #define FOO_EXTERN extern "C"
#else
    #define FOO_EXTERN extern
#endif

Before this change:

After:

The two lines where I added else and elif are not strictly necessary for this patch to work, but since there is an endif I thought I was a good idea to have them there too. I'm not entirely sure where endif is there to be honest, this rule seems to work well without it.

[sryze]

I realized that this doesn't fix the bug - instead of matching meta.extern-block.cpp the extern "C" part now matches storage.modifier.c, i.e. same as plain extern (therefore the broken tests).

Including those elif and else actually helped a little, but didn't fix the issue. I checked how the "C" part affects the output of the tokenizer and found that it parses #else as invalid.illegal.stray-else.c, i.e. as far as I understand it can't find a matching #if.

[{
        value: '#',
        scopes: ['source.cpp', 'meta.preprocessor.macro.c', 'keyword.control.directive.define.c', 'punctuation.definition.directive.c']
    }, {
        value: 'define',
        scopes: ['source.cpp', 'meta.preprocessor.macro.c', 'keyword.control.directive.define.c']
    }, {
        value: ' ',
        scopes: ['source.cpp', 'meta.preprocessor.macro.c']
    }, {
        value: 'EXTERN',
        scopes: ['source.cpp', 'meta.preprocessor.macro.c', 'entity.name.function.preprocessor.c']
    }, {
        value: ' ',
        scopes: ['source.cpp', 'meta.preprocessor.macro.c']
    }, {
        value: 'extern',
        scopes: ['source.cpp', 'meta.preprocessor.macro.c', 'meta.extern-block.cpp', 'storage.modifier.cpp']
    }, {
        value: ' ',
        scopes: ['source.cpp', 'meta.preprocessor.macro.c', 'meta.extern-block.cpp']
    }, {
        value: '"',
        scopes: ['source.cpp', 'meta.preprocessor.macro.c', 'meta.extern-block.cpp', 'string.quoted.double.cpp', 'punctuation.definition.string.begin.cpp']
    }, {
        value: 'C',
        scopes: ['source.cpp', 'meta.preprocessor.macro.c', 'meta.extern-block.cpp', 'string.quoted.double.cpp']
    }, {
        value: '"',
        scopes: ['source.cpp', 'meta.preprocessor.macro.c', 'meta.extern-block.cpp', 'string.quoted.double.cpp', 'punctuation.definition.string.end.cpp']
    }]