Adding `min/max/fit-content` and `fit-content()` values to `flex-basis` and `flex`

[yisibl]

• MDN: https://developer.mozilla.org/en-US/docs/Web/CSS/flex-basis
• Chrome status: https://chromestatus.com/feature/5635933158244352

[yisibl]

@LeaVerou PTAL, thanks!

[SebastianZ]

Btw. regarding the state of the fit-content keyword vs. the fit-content() function, I just filed https://github.com/w3c/csswg-drafts/issues/10601.

Sebastian