Add stricter lint package

[lastradev]

Hi! I really liked your project, it's very simple and useful. However, I would like it more with stricter rules.
I changed the linting package to the most used in pub.dev and made several changes so that the compiler and the linting server didn't complain
Including:

• Changing variables to constants
• Specifying the types of variables in functions and parameters
• Adding commas to style the code
• Changing functions from asynchronous to regular where not needed
• etc.

You can test it and verify that everything works correctly. However, I recognize that these rules can be subjective and if you prefer to keep the project with the old rules, there would be no problem! I would only appreciate a hacktoberfest-accepted tag.

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[lastradev]

@googlebot I signed it!