feat: add preview functionality to user commands

Adds a Lua-only `preview` flag to user commands which allows the command to be incrementally previewed like `:substitute` when 'inccommand' is set.

src/nvim/globals.h

@@ -638,6 +638,9 @@ EXTERN int motion_force INIT(=0);       // motion force for pending operator
 EXTERN bool exmode_active INIT(= false);  // true if Ex mode is active
 EXTERN bool ex_no_reprint INIT(=false);   // No need to print after z or p.
 
+// 'inccommand' command preview state
+EXTERN bool cmdpreview INIT(= false);
+
 EXTERN int reg_recording INIT(= 0);     // register for recording  or zero
 EXTERN int reg_executing INIT(= 0);     // register being executed or zero
 // Flag set when peeking a character and found the end of executed register