LibWeb: Add parent classes for managing GridTrackSizes

Add classes ExplicitTrackSizing and MetaGridTrackSize which will allow
for managing properties like auto-fill and minmax.

In the following CSS example there are 3 classes that will be used:
grid-template-column: repeat(auto-fill, minmax(50px, 1fr) 75px);

ExplicitTrackSizing - will contain the entire value. e.g.
repeat(auto-fill, minmax(50px, 1fr) 75px)

With a flag if it's a repeat, as well as references to the
MetaGridTrackSizes which is the next step down.

MetaGridTrackSize:
Contain the individual grid track sizes. Here there are two:
minmax(50px, 1fr) as well as 75px.

This way can keep track if it's a minmax function or not, and the
references to both GridTrackSizes in the case it is, or in just the one
if it is not.

GridTrackSize:
Is the most basic element, in this case there are three in total; two of
which are held by the first MetaGridTrackSize, and the third is held by
the second MetaGridTrackSize.
Examples: 50px, 1fr and 75px.

Userland/Libraries/LibWeb/Forward.h

@@ -50,6 +50,7 @@ class CSSStyleSheet;
 class CSSSupportsRule;
 class Display;
 class ElementInlineCSSStyleDeclaration;
+class ExplicitTrackSizing;
 class FilterValueListStyleValue;
 class FlexFlowStyleValue;
 class FlexStyleValue;
@@ -78,6 +79,7 @@ class MediaList;
 class MediaQuery;
 class MediaQueryList;
 class MediaQueryListEvent;
+class MetaGridTrackSize;
 class Number;
 class NumericStyleValue;
 class OverflowStyleValue;