[code-infra] Convert remaining @mui/system .js+.d.ts pairs to TypeScript (part 2)

Completes the @mui/system TypeScript conversion started in mui#48578. Collapses
the 9 remaining hand-written .js + .d.ts pairs into single .ts/.tsx source
(tsc-emitted declarations) and renames the standalone CSSProperties.d.ts:

- RtlProvider/index, colorManipulator, createStyled, createTheme/{createTheme,index},
  styleFunctionSx/{styleFunctionSx,index}, cssVars/createCssVarsProvider
- CSSProperties.d.ts -> CSSProperties.ts (type-only csstype import)

Pragmatic conversions follow the ts-package-migration skill: as any / as unknown
as T casts Babel strips (never restructuring runtime), Pattern A propTypes
(Identifier LHS) so the production guard survives, and private_createBreakpoints
kept at runtime but @internal (stripInternal) so the published type surface is
unchanged.

Tooling/config updates required by the conversion:
- scripts/coreTypeScriptProjects.js: point the api-docs-builder's @mui/system
  entry at src/index.ts (was src/index.d.ts, which the conversion removes) so
  pnpm proptypes (test_static) can build the project.
- @mui/material-pigment-css imports @mui/system/RtlProvider (now .tsx) and builds
  via tsc; added the @mui/system project reference to its tsconfig.build.json.
- exports map: drop redundant ./createTheme + ./styleFunctionSx entries (wildcard
  covers them as .ts), keep explicit ./RtlProvider pointing at index.tsx.

Documented non-breaking type-surface deltas: unstable_resolveBreakpointValues is
now in the root .d.ts (real runtime export consumed by @mui/lab Masonry, missing
from the hand-written index.d.ts); names the old index.d.ts claimed as values via
export * but index.js never exported at runtime (breakpointKeys, getStyleValue2,
extendSxProp, prepareCssVars, ...) are now correctly type-only at the root.

Verified: @mui/system build + typescript + test (1112 pass) + proptypes +
module augmentation, and downstream builds of @mui/material, @mui/lab,
@mui/material-pigment-css.

Author: Janpot

packages/mui-material-pigment-css/tsconfig.build.json

@@ -9,5 +9,6 @@
     "rootDir": "./src"
   },
   "include": ["./src/**/*.ts*"],
-  "exclude": ["src/**/*.spec.ts*", "src/**/*.test.ts*"]
+  "exclude": ["src/**/*.spec.ts*", "src/**/*.test.ts*"],
+  "references": [{ "path": "../mui-system/tsconfig.build.json" }]
 }

packages/mui-system/package.json

@@ -82,10 +82,8 @@
     "node": ">=14.0.0"
   },
   "exports": {
-    ".": "./src/index.js",
-    "./createTheme": "./src/createTheme/index.js",
-    "./RtlProvider": "./src/RtlProvider/index.js",
-    "./styleFunctionSx": "./src/styleFunctionSx/index.js",
+    ".": "./src/index.ts",
+    "./RtlProvider": "./src/RtlProvider/index.tsx",
     "./*": "./src/*/index.ts"
   }
 }

…ckages/mui-system/src/CSSProperties.d.ts packages/mui-system/src/CSSProperties.tspackages/mui-system/src/CSSProperties.d.ts renamed to packages/mui-system/src/CSSProperties.ts packages/mui-system/src/CSSProperties.d.ts renamed to packages/mui-system/src/CSSProperties.ts

@@ -1,4 +1,4 @@
-import * as CSS from 'csstype';
+import type * as CSS from 'csstype';
 
 /**
  * All non-vendor-prefixed CSS properties. (Also allows `number` in order to support CSS-in-JS libs,

packages/mui-system/src/RtlProvider/index.d.ts

@@ -2,20 +2,25 @@
 import * as React from 'react';
 import PropTypes from 'prop-types';
 
-const RtlContext = React.createContext();
+interface RtlProviderProps {
+  children?: React.ReactNode;
+  value?: boolean | undefined;
+}
+
+const RtlContext = React.createContext<boolean | undefined>(undefined);
 
-function RtlProvider({ value, ...props }) {
+function RtlProvider({ value, ...props }: RtlProviderProps) {
   return <RtlContext.Provider value={value ?? true} {...props} />;
 }
 
 RtlProvider.propTypes = {
   children: PropTypes.node,
   value: PropTypes.bool,
-};
+} as any;
 
 export const useRtl = () => {
   const value = React.useContext(RtlContext);
   return value ?? false;
 };
 
-export default RtlProvider;
+export default RtlProvider as React.FC<RtlProviderProps>;