LibWeb: Express intrinsic size layout via size constraints

Previously, we had three layout modes:

- Normal:
    - Everything uses the computed values from CSS.

- MinContent:
    - Containing blocks act as if they have 0 width.
    - All line breaking opportunities are taken.

- MaxContent:
    - Containing blocks act as if they have infinite width.
    - Only forced line breaks are accepted.

The above was based on a set of misunderstandings of CSS sizing.
A major problem with the above was that *all* containing blocks
behaved differently during intrinsic size layout, not just the
relevant one.

With this patch there are only two layout modes:

- Normal:
    - Everything uses the computed values from CSS.

- IntrinsicSizeDetermination:
    - One or more boxes have size constraints applied.

There are two size constraints per layout box, set here:

- FormattingState::NodeState::width_constraint
- FormattingState::NodeState::height_constraint

They are of type SizeConstraint and can be one of None, MinContent,
or MaxContent. The default is None.

When performing an IntrinsicSizeDetermination layout, we now assign
a size constraint to the box we're trying to determine the intrinsic
size of, which is then honored by using two new helpers to query
the dimensions of containing blocks:

- FormattingContext::containing_block_width_for(Box)
- FormattingContext::containing_block_height_for(Box)

If there's a relevant constraint in effect on the Box, the size of
its containing block is adjusted accordingly.

This is essentially an implementation of the "available space"
constraints from CSS-SIZING-3. I'm sure some things will break from
this, and we'll have to deal with that separately.

Spec: https://drafts.csswg.org/css-sizing-3/#available

Userland/Libraries/LibWeb/Layout/FormattingState.h

@@ -14,6 +14,12 @@
 
 namespace Web::Layout {
 
+enum class SizeConstraint {
+    None,
+    MinContent,
+    MaxContent,
+};
+
 struct FormattingState {
     FormattingState()
         : m_root(*this)
@@ -39,6 +45,9 @@ struct FormattingState {
         float content_height { 0 };
         Gfx::FloatPoint offset;
 
+        SizeConstraint width_constraint { SizeConstraint::None };
+        SizeConstraint height_constraint { SizeConstraint::None };
+
         float margin_left { 0 };
         float margin_right { 0 };
         float margin_top { 0 };