Improve readability of C language constants (#283)

Improve readability of C language constants

To improve the readability of C language constants, defined by the
`cConstant` syntax highlighting group [1], these are now colored with
`nord9` for the foreground to make them stand out. This is important in
C, since interesting things are usually happening in their proximity,
like checking/returning an error, passing particular values/flags to
functions and so on.

[1]: https://github.com/vim/vim/blob/0e6adf8a29d5c2c96c42cc7157f71bf22c2ad471/runtime/syntax/c.vim#L313-L375



Co-authored-by: Arctic Ice Studio <development@arcticicestudio.com>
Co-authored-by: Sven Greb <development@svengreb.de>

GH-283

Author: jvoisin

colors/nord.vim

@@ -366,6 +366,7 @@ hi! link awkVariables Identifier
 call s:hi("cIncluded", s:nord7_gui, "", s:nord7_term, "", "", "")
 hi! link cOperator Operator
 hi! link cPreCondit PreCondit
+hi! link cConstant Type
 
 call s:hi("cmakeGeneratorExpression", s:nord10_gui, "", s:nord10_term, "", "", "")