Simplify UIManagerBinding::get host-function dispatch

Summary:
`UIManagerBinding::get` (CCN 71) builds the UIManager JSI host object via one long chain of `if (methodName == "...") return Function::createFromHostFunction(...)`. This is a pure, behavior-preserving refactor: contiguous groups of methods are extracted into file-local `static` helpers that each return `std::optional<jsi::Value>` (node management, child set, layout/command, measurement, DOM-compat, view-transition, etc.); `get` consults them in order and falls through identically. Method names, arities, and host-function bodies are unchanged. Only the `.cpp` is touched; no header or public API change.

Changelog: [Internal]

Differential Revision: D108027814

Author: sammy-SC