flutter_package / fluttertpc_get

Fix missing .obs for list and map

Fixes and features

- Change GetPageBuilder to WidgetCallback typedef, so we can use it outside Get and the name makes more sense.
- Added isDesktop/isMobile
- Added GetPlatform (web) getters to detect the OS it runs (using navigator UA)
- Added nil extension (can be useless now), to enforce the null type safety in JS for old dart2js versions (was undefined)

Missing code block end mark fix in README.pl.md

docs(english): translating recent pt-br changes in docs to english

also added the docs in the spanish doc

Rx equality fix

- fixed types in equality operator, added hashCode override to return rx.value.

docs changes

Update README-es.md

- fixed code sample in simple_builder.dart
- Added docs for ValueBuilder and ObxValue

General cleanup, more docs.