Add flexbox-enabled CSS to v4 /dist

[ghost]

For those who don't want to download and compile the SCSS files it'll be useful to include something like bootstrap-flexbox.css in the dist folder.

[arechsteiner]

With Microsoft dropping support for IE10 and lower in two months (source), I'd even argue to make flexbox the new default and additionally provide a bootstrap-noflexbox.css in the dist folder.

[mdo]

For those who don't want to download and compile the SCSS files it'll be useful to include something like bootstrap-flexbox.css in the dist folder.

That could be interesting. I think I had these files being generated at some point, but hated the extra noise it added to all our diffs.

I'd even argue to make flexbox the new default and additionally provide a bootstrap-noflexbox.css in the dist folder.

We won't be doing that.

[cvrebert]

Additionally, caniuse recently downgraded its rating of IE11's flexbox support due to how buggy it is.

[arechsteiner]

We won't be doing that.

@mdo: Could you elaborate why? I'm not trying to argue, but I'm curious what the reasoning is.

[cvrebert]

Bootstrap v4 currently aims to be compatible with IE9+ out of the box. (This ratchets the minimum IE version up one notch from v3, which supported IE8+.)

[GitMiester]

This would be so useful right now. The build system can be a challenge and a barrier to using this awesome flexbox file.

Edit: Also if anyone gets stuck in this situation heres a flexbox mirror in the mean time https://github.com/GitMiester/bootstrap-4-flexbox

[mdo]

Exploring this in #20682.