Formally deprecate register_widget()

[cpsievert]

Summary

Closes #127.

register_widget() doesn't follow the Shiny rendering pattern (calling a register method inside the server body doesn't read as "this will be rendered to the UI"), and its docstring has said "Deprecated. Use @render_widget instead." for a while — but no actual warning was emitted, so users had no signal at runtime.

Now it emits a deprecation warning (ShinyDeprecationWarning from shiny, falling back to DeprecationWarning on older shiny versions) pointing users at the @render_widget pattern:

# Deprecated
map_widget = register_widget("map", map)

# Recommended
@render_widget
def map():
    return map_val

Changes

• shinywidgets/_shinywidgets.py: register_widget() now warns on every call.
• tests/unit/test_widget_bootstrap.py: existing register_widget tests assert the warning; added a dedicated deprecation test.
• CHANGELOG.md: new Deprecations section entry.

Verification

• make test-unit — 151 passed, 1 xfailed
• make pyright — 0 errors
• make py-check-format — clean