patch 8.2.2182: Vim9: value of 'magic' is still relevant

Problem:    Vim9: value of 'magic' is still relevant.
Solution:   Always behave like 'magic' is on in Vim9 script (closes #7509)

src/structs.h

@@ -4309,3 +4309,9 @@ typedef struct
 // We have to guess how much a sequence of bytes may expand when converting
 // with iconv() to be able to allocate a buffer.
 #define ICONV_MULT 8
+
+typedef enum {
+    MAGIC_NOT_SET,	// p_magic not overruled
+    MAGIC_ON,		// magic on inside regexp
+    MAGIC_OFF		// magic off inside regexp
+} magic_T;