refactor: combine regexp files

regext_bt.c and regexp_nfa.c are inlined into regexp.c instead of
included as a header. This makes developer tools like clang-tidy and
clangd be able to understand the code better.

src/nvim/regexp.h

@@ -17,12 +17,8 @@
 #define REX_USE        2       ///< to allow \z\1 et al.
 #define REX_ALL       (REX_SET | REX_USE)
 
-// regexp.c
-// uncrustify:off
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "regexp.h.generated.h"
-# include "regexp_bt.h.generated.h"
 #endif
-// uncrustify:on
 
 #endif  // NVIM_REGEXP_H