schema: Make modifiers plural in LabelSchema

[radeksimko]

This is to enable hashicorp/terraform-schema#96

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The PR introduces a number of changes:

• SemanticTokenModifier in LabelSchema becomes plural, allowing assignment of more than 1 modifier via schema
• SemanticTokenModifier in BlockSchema becomes plural, allowing assignment of more than 1 modifier via schema (this actually remains unused downstream in Declare all token modifiers explicitly for labels and attributes terraform-schema#100 but it's mostly just to align the API and make it more flexible 🤷🏻
• Introduce SemanticTokenModifiers to AttrSchema such that we can use it downstream to report dependent attributes and later for adding any more modifiers per Provide token modifiers for attributes terraform-ls#835
• Removes auto-assignment of the dependent modifier, to apply the "principle of least surprise". Declare all token modifiers explicitly for labels and attributes terraform-schema#100 basically reintroduces these modifiers more explicitly - which can be verbose but I still believe explicit & verbose is better than brief and magic.

[dbanck]

LGTM

Thanks for splitting this into four easy to follow commits