Add some more lint rules; following those in the Angular repo (dart-lang/markdown#221)

pkgs/markdown/analysis_options.yaml

diff --git a/pkgs/markdown/analysis_options.yaml b/pkgs/markdown/analysis_options.yaml
index 9d40fb5..58a1e86 100644
--- a/pkgs/markdown/analysis_options.yaml
+++ b/pkgs/markdown/analysis_options.yaml
@@ -12,6 +12,7 @@
     #- annotate_overrides
     - avoid_empty_else
     - avoid_init_to_null
+    - avoid_null_checks_in_equality_operators
     - avoid_return_types_on_setters
     - await_only_futures
     - camel_case_types
@@ -23,18 +24,23 @@
     - empty_statements
     - hash_and_equals
     - implementation_imports
+    - iterable_contains_unrelated_type
     - library_names
     - library_prefixes
+    - list_remove_unrelated_type
     - non_constant_identifier_names
     - omit_local_variable_types
     - only_throw_errors
+    - overridden_fields
+    - package_api_docs
     - prefer_final_fields
     - prefer_is_not_empty
     #- prefer_single_quotes
     - slash_for_doc_comments
-    - test_types_in_equals
+    - super_goes_last
     - test_types_in_equals
     - throw_in_finally
     - type_init_formals
+    - unnecessary_brace_in_string_interps
     - unrelated_type_equality_checks
     - valid_regexps