Phantom types: add most missing flexbox functions/values

[rlefevre]

Hi, first PR here so feedback is welcome.

Important notes / feedback likely needed

1. I replaced displayFlex by display flex_ as it seems that underscore won the namespace war for now. Tell me if I'm wrong.
2. I included a fix to "close" the "open" record parameter of alignItems and all (in passing) as they seemed wrong (their value record was parameterized on accepts). But I may have misunderstood something so tell me if I should remove this modification.
3. I did not include yet overflow and baseline alignment for justifyContent and alignContent as they are still in a draft specification and are not well supported by all browsers. However alignItems already implements them partially (overflow alignment is limited to safe center or unsafe center but center | start | end | self-start | self-end | flex-start | flex-end should also be valid). So I need feedback to know what to do.
4. I also did not include intrinsic sizing (min-content, max-content, ...) for flexBasis for the same reasons. However they are listed in the phantom-types issue, so I'm not really sure what we want.

Other less important notes

• I added a 3rd level of documentation to try to have a nice flexbox section. If this is not wanted, I can flatten out the flexbox documentation to 2 levels as the rest of the document.
• I renamed flexFlow1 (in the listed function to implement) to flexFlow as it seemed more consistent with other multiple values functions
• I struggled with the functions definition order. Maybe something should be added in the guidelines about that or a final pass done once all types are implemented.
• They are some inconsistency between comments about the use of values terminated by an underscore in the source. I choose what seemed the best but a pass to harmonize them is most likely needed at the end.

Thank you very much for this library.
Regards

[brianvanburken]

Awesome work! nowrap is all lowercase as a value. Should we be consistent with the values and make this also all lowercase?

[rlefevre]

Actually, nowrap already existed for white-space (I missed it), so I removed noWrap and added an example in nowrap.

Thank you!