refactor(options): remove option type macros

Problem: We have `P_(BOOL|NUM|STRING)` macros to represent an option's type, which is redundant because `OptValType` can already do that. The current implementation of option type flags is also too limited to allow adding multitype options in the future.

Solution: Remove `P_(BOOL|NUM|STRING)` and replace it with a new `type_flags` attribute in `vimoption_T`. Also do some groundwork for adding multitype options in the future.

Side-effects: Attempting to set an invalid keycode option (e.g. `set t_foo=123`) no longer gives an error.

src/nvim/memory.c

@@ -515,6 +515,13 @@ bool strequal(const char *a, const char *b)
   return (a == NULL && b == NULL) || (a && b && strcmp(a, b) == 0);
 }
 
+/// Returns true if first `n` characters of strings `a` and `b` are equal. Arguments may be NULL.
+bool strnequal(const char *a, const char *b, size_t n)
+  FUNC_ATTR_PURE FUNC_ATTR_WARN_UNUSED_RESULT
+{
+  return (a == NULL && b == NULL) || (a && b && strncmp(a, b, n) == 0);
+}
+
 // Avoid repeating the error message many times (they take 1 second each).
 // Did_outofmem_msg is reset when a character is read.
 void do_outofmem_msg(size_t size)