TSL: Add layout type for Fn

[linbingquan]

As the title says.

[linbingquan]

@Methuselah96 The Pr code should be fine. CI reported an error, but I don't know how to handle it

Error in itowns
Error: 
/home/runner/work/three-ts-types/three-ts-types/DefinitelyTyped/types/itowns/test/3DTiles25D.ts
  2:24  error  [email protected], 5.2, 5.3, 5.4, 5.5, 5.6, 5.7, 5.8, 5.9 compile error: 
Cannot find module 'proj4' or its corresponding type declarations  @definitelytyped/expect

✖ 1 problem (1 error, 0 warnings)

[linbingquan]

Same CI error in https://github.com/DefinitelyTyped/DefinitelyTyped/actions/runs/15985232481/job/45088283125?pr=73156

[Methuselah96]

Superseded by #1833