Add string value definition

[MariaSolOs]

Adds the definition of StringValue, which is currently used in the metamodel by inline completions.

[MariaSolOs]

@dbaeumer I think we should remove the insertTextFormat property from InlineCompletionItem:

language-server-protocol/_specifications/lsp/3.18/language/inlineCompletion.md

Lines 233 to 237 in a1766bb

	/**
	* The format of the insert text. The format applies to the `insertText`.
	* If omitted, defaults to `InsertTextFormat.PlainText`.
	*/
	insertTextFormat?: InsertTextFormat;

Why?

• It becomes redundant with the type of insertText being string | StringValue.
• The metamodel doesn't have such property.
• The analogous VS Code definition doesn't have such property either

[dbaeumer]

@MariaSolOs makes perfekt sense to remove insertTextFormat

[MariaSolOs]

@MariaSolOs makes perfekt sense to remove insertTextFormat

Alright, doing that here: #1894 :)