feat(ui): expose onMount to CodeField

[AlessioGr]

This is useful for modifying the monaco instance, e.g. to add external types to the editor:

<CodeField
        autoComplete={autoComplete}
        field={props}
        forceRender={forceRender}
        key={key}
        onMount={(editor, monaco) => {
          console.log('editor mounted')
          monaco.languages.typescript.typescriptDefaults.setCompilerOptions({
            allowNonTsExtensions: true,
            moduleResolution: monaco.languages.typescript.ModuleResolutionKind.NodeJs,
            paths: {
              payload: ['file:///node_modules/payload/index.d.ts'],
            },
            target: monaco.languages.typescript.ScriptTarget.ESNext,
            typeRoots: ['node_modules/@types', 'node_modules/payload'],
          })
          const run = async () => {
            const types = await fetch('https://unpkg.com/payload@latest/dist/index.d.ts')
            const typesText = await types.text()
            monaco.languages.typescript.typescriptDefaults.addExtraLib(
              typesText,
              'file:///node_modules/payload/index.d.ts',
            )
          }
          void run()
        }}
        path={path}
        permissions={permissions}
        readOnly={readOnly}
        renderedBlocks={renderedBlocks}
        schemaPath={schemaPath}
        validate={validate}
      />