Add ignoreFunctions: [] to length-zero-no-unit

[JuanFML]

Added condition to ignore when function is detected and a test to check it with.

Which issue, if any, is this issue related to?

Fixes issue #4945

Is there anything in the PR that needs further explanation?

I left some comments in the code as it is another type of solution, this is my first PR, so please let me know if the solution wasn't what you were looking for, in order to fix it. There would be still left to add to the README.md file the new option added, although I wanted to get it checked first before adding it.

[vankop]

Thanks for your PR, some comments:

[vankop]

@JuanFML I have reworked things a little bit, take a look

[JuanFML]

@vankop Yeah they work perfectly your changes, thanks a lot! I don't have to commit again do I? With your commit is all done?

[vankop]

I don't have to commit again do I?

yes, all is fine

[hudochenkov]

Thank you for working on this!

Could you add test cases, please?

/* it should report only first 0px */
a { margin: 0px var(--prop, 0px) }

/* it should report only second 0px */
a { margin: calc(5px + var(--prop, 0px)) 0px }

[jeddy3]

Revisiting after the refactoring in #5256.

I think a simple conditional just after line 52 should work now:

if (isFunction(valueNode) && optionsMatches(secondary, 'ignoreFunctions', value)) return false;

With the following tests:

testRule({
	ruleName,
	config: [true, { ignoreFunctions: ['var', /^--/] }],
	accept: [
		{
			code: 'a { top: var(--foo, 0px); }',
		},
		{
			code: 'a { top: --bar(0px); }',
		},
	],
	reject: [
		{
			code: 'a { margin: var(--foo, 0px) 0px --bar(0px); }',
			fixed: 'a { margin: var(--foo, 0px) 0 --bar(0px); }',

			message: messages.rejected,
			line: 1,
			column: 30,
		},
	],
});

We'll need to require lodash on line 5:

const _ = require('lodash');

And update the options validator on line 41:

ignoreFunctions: [_.isString, _.isRegExp],

And add a isFunction function on line 154:

function isFunction({ type }) {
	return type === 'function';
}

The docs can look like this, e.g.

Given:

["var", "/^--/"]

The following patterns are not considered violations:

a { top: var(--foo, 0px); }

a { top: --bar(0px); }

@JuanFML Would you like to update the pull request?