Support content_for "block" type completion

[charlespwd]

We want block type completion

{% content_for "block", type: "█" %}
{% content_for "block", type: "foo█" %}

Steps:

• Create a ContentForBlockTypeCompletionProvider in theme-language-server-common/src/completions/providers
• Import it and add an instance to the list of providers in theme-language-server-common/src/completions/CompletionProvider
• Logic should be something like
  • Early return unless
    • the node.type === NodeTypes.String
    • the parentNode.type === NodeTypes.NamedArgument && its name is === 'type'
    • the grandParentNode.type === NodeTypes.ContentForMarkup && its contentForType is === 'block'
  • Return all the options from the block schema registry (depends on Add section & block schema registries for use in the language server and theme checks #572)