Introduce `ObjectModel` namespace for elements within CSSOM

[JakeQZ]

Following from the discussion in #1542...

Refs:

• CSS Object Model (CSSOM) - MDN;
• CSS Typed Object Model API - MDN.

We can move all classes/interfaces that represent CSS content into an additional root-level namespace in the package, Sabberworm\CSS\ObjectModel. This will help separate the content classes from the settings/parsing/rendering classes. While doing so, we can rename some to more accurately describe what they represent, according to the formal grammar. The use of class_alias() will allow the renaming/moving of classes without a breaking change.

Current proposed move/renames are as follows:

• CSSElement ➜ ObjectModel\CssElement – (helper class/interface)
• RuleSet\AtRuleSet ➜ ObjectModel\Rule\BlockAtRule – (https://drafts.csswg.org/css-syntax/#at-rules)
• RuleSet\DeclarationBlock ➜ ObjectModel\Rule\RuleSet – (https://drafts.csswg.org/css2/#rule-sets)
• RuleSet\DeclarationList ➜ ObjectModel\Rule\DeclarationList – (https://drafts.csswg.org/css-syntax/#block-contents)
• RuleSet\LegacyDeclarationListMethods ➜ ObjectModel\Rule\LegacyDeclarationListMethods – (helper class/interface)
• RuleSet\RuleSet ➜ ObjectModel\Rule\DeclarationBlock – (https://drafts.csswg.org/cssom/#css-declaration-blocks)

This is not yet a complete list. This OP is a live document and can be edited and added to.