style::Display is not according to specifications

[Weibye]

Our current Display-style looks like this

struct Display {
    Flex,
    None
}

This definition does not actually exist in the specification. As far as I can tell, these are two separate properties:

• Inner Display Layout Model: display-inside
  https://www.w3.org/TR/css-display-3/#box-generation
  Which defines what layout model to use (flexbox vs css-grid vs flow vs table ++)
• Box Generation: display-box
  https://www.w3.org/TR/css-display-3/#valdef-display-none
  Which defines if the flex-item will generate any boxes at all

I think it makes more sense to define the two different properties according to specifications as two separate enums, to be controlled or improved upon independently

Originally posted by @Weibye in #96 (comment)

[alice-i-cecile]

Related to #28.

[nicoburns]

This property does exist in the specification (https://www.w3.org/TR/css-display-3/#the-display-properties). The specification defines it as:

<display> = [ <display-outside> || <display-inside> ] | <display-listitem> | <display-internal> | <display-box> | <display-legacy>

<display-outside>  = block | inline | run-in
<display-inside>   = flow | flow-root | table | flex | grid | ruby
<display-listitem> = <display-outside>? && [ flow | flow-root ]? && list-item
<display-internal> = table-row-group | table-header-group |
                     table-footer-group | table-row | table-cell |
                     table-column-group | table-column | table-caption |
                     ruby-base | ruby-text | ruby-base-container |
                     ruby-text-container
<display-box>      = contents | none
<display-legacy>   = inline-block | inline-table | inline-flex | inline-grid

Where:

• | means "or"
• && means "and"
• ? means optional
• || means "concatenate"

This specification does include some cases where you need to specify more than one keyword however those cases are only hit in the case of display: list-item, or when specifying both a <display-inside> and <display-outside> value. But as we don't support display: list-item and only block as the display-outside value we do not need to support these cases, and can simplify this specification down to a flat enum.