refactor(options): reduce findoption() usage

Problem: Many places in the code use `findoption()` to access an option using its name, even if the option index is available. This is very slow because it requires looping through the options array over and over.

Solution: Use option index instead of name wherever possible. Also introduce an `OptIndex` enum which contains the index for every option as enum constants, this eliminates the need to pass static option names as strings.

src/nvim/option_defs.h

@@ -122,3 +122,8 @@ typedef enum {
   kOptReqWin    = 1,  ///< Request window-local option value
   kOptReqBuf    = 2,  ///< Request buffer-local option value
 } OptReqScope;
+
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+// Initialize the OptIndex enum.
+# include "options_enum.generated.h"
+#endif