refactor(options): option flags enum #30961

Problem: Currently we use macros with hardcoded flag values for option flags, which is messy and requires a lot of mental math for adding / removing option flags. Using macros for option flags also means that they cannot be used inside debuggers.

Solution: Create a new `OptFlags` enum that stores all the option flags in an organized way that is easier to understand.

src/nvim/cmdexpand.c

@@ -1350,7 +1350,7 @@ char *addstar(char *fname, size_t len, int context)
 ///                          it.
 ///  EXPAND_BUFFERS          Complete file names for :buf and :sbuf commands.
 ///  EXPAND_FILES            After command with EX_XFILE set, or after setting
-///                          with P_EXPAND set.  eg :e ^I, :w>>^I
+///                          with kOptFlagExpand set.  eg :e ^I, :w>>^I
 ///  EXPAND_DIRECTORIES      In some cases this is used instead of the latter
 ///                          when we know only directories are of interest.
 ///                          E.g.  :set dir=^I  and  :cd ^I