fix(react-19): switch JSX.Element to React.ReactElement to support React 19 and 18

[wdcryer]

As of React 19, the JSX namespace is no longer global. It now uses React.JSX.
https://react.dev/blog/2024/04/25/react-19-upgrade-guide#the-jsx-namespace-in-typescript

As a result, usages of JSX.Element in this library throw type errors:

ERROR in ./node_modules/react-complex-tree/src/types.ts:367:14
TS2503: Cannot find namespace 'JSX'.
    365 |   C extends string = never
    366 | > extends TreeEnvironmentConfiguration<T, C> {
  > 367 |   children?: JSX.Element | (JSX.Element | null)[] | null;
        |              ^^^
    368 | }
    369 |
    370 | export interface UncontrolledTreeEnvironmentProps<

ERROR in ./node_modules/react-complex-tree/src/types.ts:367:29
TS2503: Cannot find namespace 'JSX'.
    365 |   C extends string = never
    366 | > extends TreeEnvironmentConfiguration<T, C> {
  > 367 |   children?: JSX.Element | (JSX.Element | null)[] | null;
        |                             ^^^
    368 | }
    369 |
    370 | export interface UncontrolledTreeEnvironmentProps<

ERROR in ./node_modules/react-complex-tree/src/types.ts:381:13
TS2503: Cannot find namespace 'JSX'.
    379 |   liveDescriptors?: LiveDescriptors;
    380 |   getItemTitle: (item: TreeItem<T>) => string;
  > 381 |   children: JSX.Element | (JSX.Element | null)[] | null;
        |             ^^^
    382 |   disableMultiselect?: boolean;
    383 | }
    384 |

ERROR in ./node_modules/react-complex-tree/src/types.ts:381:28
TS2503: Cannot find namespace 'JSX'.
    379 |   liveDescriptors?: LiveDescriptors;
    380 |   getItemTitle: (item: TreeItem<T>) => string;
  > 381 |   children: JSX.Element | (JSX.Element | null)[] | null;
        |                            ^^^
    382 |   disableMultiselect?: boolean;
    383 | }
    384 |

In @types/react, React.ReactElement appears to be equivalent to JSX.Element in v18 and and React.JSX.Element in v19. Using that type in place of JSX.Element should work in both React versions.
v18: https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/react/v18/index.d.ts#L4363
v19: https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/react/index.d.ts#L4077

Unfortunately I get errors when running yarn build so this should be checked before merging. Perhaps I should be using an older version of Yarn. I have the latest version and it seems to modify the lock file dramatically.

Solves #434

@lukasbach

[lukasbach]

Thanks for the contribution! yarn build seems to run fine in CI.

[wdcryer]

Thank you, @lukasbach!