Make {watcher} a required dependency for autoreload

[shikokuchuo]

Summary

Moves {watcher} from Suggests to Imports and uses it unconditionally for autoreload. Removes the legacy polling-based file watcher.

Changes

• DESCRIPTION: watcher → Imports, with (>= 0.2.0).
• R/shinyapp.R (initAutoReloadMonitor()): drop the is_installed("watcher") branch, the legacy observe()/invalidateLater() poller, and the shiny.autoreload.legacy_warning option. Return invisible(watcher$stop) so cleanup flows through the existing onStop → monitorHandle() path.
• R/shiny-options.R / man/shinyOptions.Rd: simplify the shiny.autoreload option docs now that watcher is the sole path (drop the historical "no longer polls"/"no longer expensive" hedging).
• NEWS.md: note the dependency change.

Compatibility

• shiny.autoreload.interval / .pattern unchanged (interval still passed as latency, default 250ms).
• shiny.autoreload.legacy_warning removed; silently ignored if set.

[shikokuchuo]

@cpsievert The original intention of #4185 was to enable this by default, and as watcher now uses R's standard build toolchain, this should address any lingering source installation concerns. End of the day, I think this is a good affordance for Shiny to provide.

[cpsievert]

Thanks!