Add the infinite_list sample (#440)

This PR adds a Flutter sample app that shows an implementation of the "infinite list" UX pattern. That is, a list is shown to the user as if it was continuous although it is internally paginated. This is a common feature of mobile apps, from shopping catalogs through search engines to social media clients.

infinite_list/analysis_options.yaml

@@ -0,0 +1,31 @@
+include: package:pedantic/analysis_options.yaml
+
+analyzer:
+  strong-mode:
+    implicit-casts: false
+    implicit-dynamic: false
+
+linter:
+  rules:
+    - avoid_types_on_closure_parameters
+    - avoid_void_async
+    - await_only_futures
+    - camel_case_types
+    - cancel_subscriptions
+    - close_sinks
+    - constant_identifier_names
+    - control_flow_in_finally
+    - directives_ordering
+    - empty_statements
+    - hash_and_equals
+    - implementation_imports
+    - non_constant_identifier_names
+    - package_api_docs
+    - package_names
+    - package_prefixed_library_names
+    - test_types_in_equals
+    - throw_in_finally
+    - unnecessary_brace_in_string_interps
+    - unnecessary_getters_setters
+    - unnecessary_new
+    - unnecessary_statements