fix(es/codegen): Fix codegen of TypeScript mapped types (#6621)

Author: kevinbarabash

crates/swc_ecma_codegen/src/typescript.rs

@@ -505,7 +505,7 @@ where
                 }
                 TruePlusMinus::Plus => {
                     punct!("+");
-                    punct!("/");
+                    punct!("?");
                 }
                 TruePlusMinus::Minus => {
                     punct!("-");

crates/swc_ecma_codegen/tests/fixture/issues/6616/input.ts

@@ -0,0 +1,3 @@
+type Custom<A> = {
+    -readonly [P in keyof A]+?: A[P];
+};

crates/swc_ecma_codegen/tests/fixture/issues/6616/output.ts

@@ -0,0 +1,3 @@
+type Custom<A> = {
+    -readonly [P in keyof A]+?: A[P];
+};