Before if an element didn't have a main min size we would clamp
it to a literal zero. If that element also had a flex-basis 0
it's width would end up being 0.
This patch adds a determine_min_main_size_of_child function that
will calculate the minimum main size for the box based on the
content of the box.
We use the result of that function now instead of clamping
the element main min size to 0.
This also adds one more box to the flex.html test page, which is
the same flex: 0 0 0 box but with flex-direction: column.
This is not just moving the code from StyleResolver to Parser. The logic
has changed to allow for the `flex-basis` to come before or after the
`flex-grow/shrink` values, as well as handle the special one-value
cases.
Also added test cases to flex.html to check the parsing. It does parse
correctly, but elements with `flex-basis: auto` do not calculate their
width correctly.
This adds test pages for border-radius, CSS custom properties and
flexboxes to the default page in the Browser.
I used those files to develop said features and they can be of use
when debugging in the future or just to showcase those features.
