Only parse safe as contextual kw in extern blocks

I don't like the party of `bool`s that is becoming, but two isn't worth a refactoring yet IMO.

crates/parser/test_data/generated/runner.rs

@@ -527,6 +527,10 @@ mod ok {
         run_and_expect_no_errors("test_data/parser/inline/ok/return_type_syntax_in_path.rs");
     }
     #[test]
+    fn safe_outside_of_extern() {
+        run_and_expect_no_errors("test_data/parser/inline/ok/safe_outside_of_extern.rs");
+    }
+    #[test]
     fn self_param() { run_and_expect_no_errors("test_data/parser/inline/ok/self_param.rs"); }
     #[test]
     fn self_param_outer_attr() {

crates/parser/test_data/parser/inline/ok/safe_outside_of_extern.rast

@@ -0,0 +1,30 @@
+SOURCE_FILE
+  FN
+    FN_KW "fn"
+    WHITESPACE " "
+    NAME
+      IDENT "foo"
+    PARAM_LIST
+      L_PAREN "("
+      R_PAREN ")"
+    WHITESPACE " "
+    BLOCK_EXPR
+      STMT_LIST
+        L_CURLY "{"
+        WHITESPACE " "
+        EXPR_STMT
+          BIN_EXPR
+            PATH_EXPR
+              PATH
+                PATH_SEGMENT
+                  NAME_REF
+                    IDENT "safe"
+            WHITESPACE " "
+            EQ "="
+            WHITESPACE " "
+            LITERAL
+              TRUE_KW "true"
+          SEMICOLON ";"
+        WHITESPACE " "
+        R_CURLY "}"
+  WHITESPACE "\n"

crates/parser/test_data/parser/inline/ok/safe_outside_of_extern.rs

@@ -0,0 +1 @@
+fn foo() { safe = true; }