Restore @babel/types support for old TS versions (#12521)

packages/babel-types/package.json

@@ -14,10 +14,11 @@
     "directory": "packages/babel-types"
   },
   "main": "lib/index.js",
+  "types": "lib/index-legacy.d.ts",
   "typesVersions": {
-    "<3.7": {
-      "lib/index.d.ts": [
-        "lib/index-ts3.7.d.ts"
+    ">=3.7": {
+      "lib/index-legacy.d.ts": [
+        "lib/index.d.ts"
       ]
     }
   },

packages/babel-types/scripts/generators/typescript-legacy.js

@@ -5,7 +5,7 @@ const stringifyValidator = require("../utils/stringifyValidator");
 const toFunctionName = require("../utils/toFunctionName");
 
 let code = `// NOTE: This file is autogenerated. Do not modify.
-// See packages/babel-types/scripts/generators/typescript-3.7.js for script used.
+// See packages/babel-types/scripts/generators/typescript-legacy.js for script used.
 
 interface BaseComment {
   value: string;