refactor(#4632): correct tabindex parsing logic

[ahmedhalac]

Ensures tabindex is correctly parsed as an integer using regex instead of parseInt to comply with HTML standards.

Related to #4632

[WilcoFiers]

This is a good start, but I think we can do a little better. Going over the code base, it looks like axe has an awful lot of places where it's parsing tabindex. I think it would be better if we put the code for this in one place. Normally we'd do this in a commons function, but because one of the places is in core, we'll need to do this in core/utils. So how about this:

In lib/core/utils/ create a file parse-tabindex.js with a parseTabindex function that accepts either a string or null. And which returns an integer if the string is a valid tabindex value, or null if not valid or if the value is null. This would need tests in a new test/core/utils/parse-tabindex.js.

This function could then be used in the following files, instead of the current ad-hoc methods:

• lib/checks/keyboard/focusable-no-name-evaluate.js
• lib/checks/keyboard/no-focusable-content-evaluate.js
• lib/checks/keyboard/tabindex-evaluate.js
• lib/commons/dom/get-tabbable-elements.js
• lib/commons/dom/inserted-into-focus-order.js
• lib/commons/dom/is-focusable.js
• lib/commons/dom/is-in-tab-order.js
• lib/core/base/context/create-frame-context.js
• lib/rules/autocomplete-matches.js
• lib/rules/no-negative-tabindex-matches.js