[babel-plugin] various TypeScript fixes

[henryqdineen]

What changed / motivation ?

We have been using skipLibCheck: true to suppress a handful TypeScript errors (see below) coming from @stylexjs/babel-plugin and we would prefer to remove this workaround. This PR attempts to fix the issue by providing .d.ts files for the problematic modules instead of relying on flow-api-translator. This is supported by the generate-types.js script.

Maintaining the d.ts files does seem like it could be a pain so I an open to other suggestions. I have considered:

• Rewriting the flow types so flow-api-translator generates correct TS (not sure I know Flow well enough)
• Implementing a codemod in postProcessTSOutput()

Thanks!

Additional Context

The errors we were seeing:

node_modules/@stylexjs/babel-plugin/lib/shared/common-types.d.ts:38:3 - error TS2411: Property '$$css' of type 'string | true' is not assignable to 'string' index type 'string | null'.

38   $$css: true | string;
     ~~~~~

node_modules/@stylexjs/babel-plugin/lib/shared/stylex-define-vars.d.ts:13:4 - error TS1337: An index signature parameter type cannot be a literal type or generic type. Consider using a mapped object type instead.

13   [$$Key$$: keyof Vars]: string;
      ~~~~~~~

node_modules/@stylexjs/babel-plugin/lib/shared/stylex-create-theme.d.ts:23:5 - error TS2411: Property '$$css' of type 'true' is not assignable to 'string' index type 'string'.

23   { $$css: true; readonly [$$Key$$: string]: string },
       ~~~~~

node_modules/@stylexjs/babel-plugin/lib/index.d.ts:53:1 - error TS2309: An export assignment cannot be used in a module with other exported elements.

53 export = $$EXPORT_DEFAULT_DECLARATION$$;
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Diff of the previous generated .d.ts files to the ones added in this PR. (some changes just from prettier):

// node_modules/@stylexjs/babel-plugin/lib/index.d.ts 
@@ -39,10 +39,10 @@
   config?:
     | boolean
     | {
-        useLayers?: boolean,
-        enableLTRRTLComments?: boolean,
-        legacyDisableLayers?: boolean,
-      }
+        useLayers?: boolean;
+        enableLTRRTLComments?: boolean;
+        legacyDisableLayers?: boolean;
+      },
 ): string;
 export type StyleXTransformObj = Readonly<{
   (): PluginObj;
@@ -50,4 +50,4 @@
   processStylexRules: typeof processStylexRules;
 }>;
 declare const $$EXPORT_DEFAULT_DECLARATION$$: StyleXTransformObj;
-export = $$EXPORT_DEFAULT_DECLARATION$$;
+export default $$EXPORT_DEFAULT_DECLARATION$$;

// node_modules/@stylexjs/babel-plugin/lib/shared/common-types.d.ts 
@@ -33,10 +33,9 @@
     | string
     | Readonly<{ [$$Key$$: string]: null | string }>;
 }>;
-export type FlatCompiledStyles = Readonly<{
-  [$$Key$$: string]: string | null;
-  $$css: true | string;
-}>;
+export type FlatCompiledStyles = Readonly<
+  { [$$Key$$: string]: string | null } & { $$css: true | string }
+>;
 export type StyleXOptions = Readonly<{
   classNamePrefix: string;
   debug: null | undefined | boolean;

// node_modules/@stylexjs/babel-plugin/lib/shared/stylex-create-theme.d.ts 
@@ -20,7 +20,7 @@
   },
   options?: StyleXOptions,
 ): [
-  { $$css: true; readonly [$$Key$$: string]: string },
+  { $$css: true } & { readonly [$$Key$$: string]: string },
   { [$$Key$$: string]: InjectableStyle },
 ];
 export default styleXCreateTheme;

// node_modules/@stylexjs/babel-plugin/lib/shared/stylex-define-vars.d.ts 
@@ -10,17 +10,17 @@
 import type { InjectableStyle, StyleXOptions } from './common-types';
 import type { VarsConfig } from './stylex-vars-utils';
 type VarsKeysWithStringValues<Vars extends VarsConfig> = Readonly<{
-  [$$Key$$: keyof Vars]: string;
+  [$$Key$$ in keyof Vars]: string;
 }>;
 type VarsObject<Vars extends VarsConfig> = Readonly<
-  Omit<VarsKeysWithStringValues<Vars>, keyof ({ __varGroupHash__: string })> & {
+  Omit<VarsKeysWithStringValues<Vars>, keyof { __varGroupHash__: string }> & {
     __varGroupHash__: string;
   }
 >;
 declare function styleXDefineVars<Vars extends VarsConfig>(
   variables: Vars,
   options: Readonly<
-    Omit<Partial<StyleXOptions>, keyof ({ exportId: string })> & {
+    Omit<Partial<StyleXOptions>, keyof { exportId: string }> & {
       exportId: string;
     }
   >,

Pre-flight checklist

• I have read the contributing guidelines
  Contribution Guidelines
• Performed a self-review of my code

[mellyeliu]

This seems fine as an initial cleanup, we can explore a broader re-writing down the line