LibWeb+WebContent: Forbid access to underlying type of CSSPixels

Although DistinctNumeric, which is supposed to abstract the underlying
type, was used to represent CSSPixels, we have a whole bunch of places
in the layout code that assume CSSPixels::value() returns a
floating-point type. This assumption makes it difficult to replace the
underlying type in CSSPixels with a non-floating type.

To make it easier to transition CSSPixels to fixed-point math, one step
we can take is to prevent access to the underlying type using value()
and instead use explicit conversions with the to_float(), to_double(),
and to_int() methods.

Userland/Libraries/LibWeb/Layout/AvailableSpace.cpp

@@ -11,7 +11,7 @@ namespace Web::Layout {
 
 AvailableSize AvailableSize::make_definite(CSSPixels value)
 {
-    VERIFY(isfinite(value.value()));
+    VERIFY(isfinite(value.to_double()));
     return AvailableSize { Type::Definite, value };
 }