Fix unit-no-unknown false positives for font-relative length units

[ybiquitous]

What steps are needed to reproduce the bug?

For example, the CSS code using the ic unit:

a { width: 10ic; }

We can reproduce the bug on the demo.

What Stylelint configuration is needed to reproduce the bug?

{
  "rules": {
    "unit-no-unknown": true
  }
}

How did you run Stylelint?

CLI with stylelint *.css

Which version of Stylelint are you using?

14.13.0

What did you expect to happen?

No problems are to be reported.

What actually happened?

The following problem is reported:

test.css
 1:14  ✖  Unexpected unknown unit "ic"  unit-no-unknown

1 problem (1 error, 0 warnings)

Does the bug relate to non-standard syntax?

No.

Proposal to fix the bug

To Update the internal list for the units:

stylelint/lib/reference/units.js

Lines 5 to 12 in eee9deb

	const lengthUnits = new Set([
	// Relative length units
	'em',
	'ex',
	'ch',
	'rem',
	'rlh',
	'lh',