Add a Lua constant for UI_STYLE_LEXER_MAX

Currently, there's no mechanism for defining user styles without risk
of collision with default lexer and theme settings.

Very few lexers use more than 10,
let alone all 64 of the allowed styles, so
UI_STYLE_LEXER_MAX - (no. of user defined styles)
allows for a reasonably large number of user-defined styles
before collision becomes a problem.

vis-lua.c

@@ -3276,6 +3276,7 @@ void vis_lua_init(Vis *vis) {
 		enum UiStyle id;
 		const char *name;
 	} styles[] = {
+		{ UI_STYLE_LEXER_MAX,         "STYLE_LEXER_MAX"         },
 		{ UI_STYLE_DEFAULT,           "STYLE_DEFAULT"           },
 		{ UI_STYLE_CURSOR,            "STYLE_CURSOR"            },
 		{ UI_STYLE_CURSOR_PRIMARY,    "STYLE_CURSOR_PRIMARY"    },