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: https://github.com/microsoft/language-server-protocol/blob/a1766bb2c70bb8e847a22ca99f933047716d9a64/_specifications/lsp/3.18/language/inlineCompletion.md?plain=1#L233-L237

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 :)