refactor: replace unnecessary helper functions in optionstr.c

Replaces unnecessary helper functions in `optionstr.c` such as
`get_option_flags()`, `get_option_fullname()`, `set_option_flag()`,
`is_global_option()`, etc. with a single `get_option()` helper function
that allows direct access to the `options` array.

Also refactors `f_exists()` to use `get_varp_scope` instead of using
`get_option_tv`. This opens up the path for removing `getoptions_T`
altogether later down the line since the hidden option logic is no
longer needed.

src/nvim/option_defs.h

@@ -990,4 +990,41 @@ typedef struct {
   uint64_t channel_id;     /// Only used when script_id is SID_API_CLIENT.
 } LastSet;
 
+// WV_ and BV_ values get typecasted to this for the "indir" field
+typedef enum {
+  PV_NONE = 0,
+  PV_MAXVAL = 0xffff,  // to avoid warnings for value out of range
+} idopt_T;
+
+typedef struct vimoption {
+  char *fullname;        // full option name
+  char *shortname;       // permissible abbreviation
+  uint32_t flags;               // see below
+  char_u *var;             // global option: pointer to variable;
+                           // window-local option: VAR_WIN;
+                           // buffer-local option: global value
+  idopt_T indir;                // global option: PV_NONE;
+                                // local option: indirect option index
+  char *def_val;         // default values for variable (neovim!!)
+  LastSet last_set;             // script in which the option was last set
+} vimoption_T;
+
+// The options that are local to a window or buffer have "indir" set to one of
+// these values.  Special values:
+// PV_NONE: global option.
+// PV_WIN is added: window-local option
+// PV_BUF is added: buffer-local option
+// PV_BOTH is added: global option which also has a local value.
+#define PV_BOTH 0x1000
+#define PV_WIN  0x2000
+#define PV_BUF  0x4000
+#define PV_MASK 0x0fff
+#define OPT_WIN(x)  (idopt_T)(PV_WIN + (int)(x))
+#define OPT_BUF(x)  (idopt_T)(PV_BUF + (int)(x))
+#define OPT_BOTH(x) (idopt_T)(PV_BOTH + (int)(x))
+
+// Options local to a window have a value local to a buffer and global to all
+// buffers.  Indicate this by setting "var" to VAR_WIN.
+#define VAR_WIN ((char_u *)-1)
+
 #endif  // NVIM_OPTION_DEFS_H