Resolves phpcs errors

[nateconley]

Description of the Change

Edits the plugin to meet phpcs linting rules.

Changes include:

• Proper escaping and sanitization
• phpdoc implementations
• Initializing class variables where applicable
• json_encode to wp_json_encode
• Strict comparisons in conditionals and in_array
• Do not use $var
• Explicitly set in_footer for scripts
• Restructure strings to be properly translatable
• Adds translators: text where placeholders exist
• Deprecates function where migrated to snake case

Addresses php linting in #12

How to test the Change

• Ensure plugin functionality is maintained.

Changelog Entry

Deprecated - mailchimpSF_signup_form -> please use mailchimp_sf_signup_form

Credits

@nateconley

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[dkotter]

A few comments to look at.

The only other thing we need to think through is we're renaming a bunch of functions to avoid snake case. I think this is fine for any functions that are meant to only be used internally but any functions that we expect to be used by others, we need to properly deprecate those so they still work correctly.

As an example, the readme mentions the use of mailchimpSF_signup_form, so I'd imagine there's a decent amount of people using this function on their site. If we don't deprecate this, any site using that function will not only not work but will throw undefined errors.

Not sure what other functions people may be using that we'll want to deprecate. If we're concerned, could also remove the linting rule about snake case and just keep function names as-is, though I like the idea of cleaning those up.

[nateconley]

@dkotter I updated based on your comments. I used _deprecated_function to deprecate the one function referenced in the documentation and used for display. We could recreate this function if you think that would be more appropriate.

[nateconley]

@dkotter Noting that this class probably should live in its own file, but I didn't think this PR was the appropriate time to move it.