Use sharezone_lints for url_launcher_extended

[nilsreichardt]

I resolved the following warnings in this PR:

Nils-MBP-13-M1-3:url_launcher_extended nils$ flutter analyze
Analyzing url_launcher_extended...                                      

   info • The local variable '_canLaunch' starts with an underscore • lib/src/url_launcher_extended.dart:70:11 •
          no_leading_underscores_for_local_identifiers
   info • The local variable '_canLaunch' starts with an underscore • lib/src/url_launcher_extended.dart:119:11 •
          no_leading_underscores_for_local_identifiers
   info • The parameter name 'o' doesn't match the name 'other' in the overridden method • lib/src/url_launcher_extended.dart:136:27 •
          avoid_renaming_method_parameters
   info • The parameter name 'o' doesn't match the name 'other' in the overridden method • lib/src/url_launcher_extended.dart:160:27 •
          avoid_renaming_method_parameters
   info • Can't use a relative path to import a library in 'lib' • test/mock_url_launcher_extended_test.dart:12:8 • avoid_relative_lib_imports

5 issues found. (ran in 0.9s)

Part of #37

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