Fix false positives for flex shorthand with 2 values in length-zero-no-unit

[Sultanidze]

Clearly describe the bug

There is false negative for 0px in the flex shorthand rule:
flex: 1 0px;

It is ambiguous situation for two values in flex shorthand property,
according to documentation:
https://developer.mozilla.org/en-US/docs/Web/CSS/flex

Two values: flex-grow | flex-basis:

flex: 1 0px;

Two values: flex-grow | flex-shrink:

flex: 1 0;

Which rule, if any, is the bug related to?

e.g. length-zero-no-unit

What code is needed to reproduce the bug?

.wrapper {
  flex: 1 0px;
}

What stylelint configuration is needed to reproduce the bug?

{
  "rules": {
    "length-zero-no-unit": true,
  }
}

Which version of stylelint are you using?

13.6.0

How are you running stylelint: CLI, PostCSS plugin, Node.js API?

via stylelint-webpack-plugin

Does the bug relate to non-standard syntax (e.g. SCSS, Less etc.)?

No

What did you expect to happen?

No length-zero-no-unit error to be flagged.
Possibly it would be good to create another rule to disallow 2 values in flex shorthand property to avoid ambiguous situations, and make intentions evident

What actually happened (e.g. what warnings or errors did you get)?

The following warnings were flagged:

Value.module.scss
 32:12  ✖  Unexpected unit   length-zero-no-unit

[jeddy3]

@Sultanidze Thanks for the report and for using the template.

I can reproduce this using the demo.

I've labelled the issue as ready to implement. Please consider contributing if you have time.

There are steps on how to fix a bug in a rule in the Developer guide.

Possibly it would be good to create another rule to disallow 2 values in flex shorthand property to avoid ambiguous situations, and make intentions evident

This sounds like a good idea. Can create a separate issue for it, please?

[Sultanidze]

@jeddy3 I'm on it.
Will create separate issue for a new rule after fixing current rule.