value type informations for expressions

[samuelstroschein]

Problem

From Paraglide-JS:

// The expressions are NonNullable<unknown> because inlang can't know the value type of an expression (yet)
export declare const currentLanguageTag: (expressions: { name: NonNullable<unknown> }) => string;

From the inlang editor:

The inlang editor can't validate variable expression input of translators yet. 

Proposal

Introduce information on the value type of a VariableReference expression.

export declare const currentLanguageTag: (expressions: { name: string }) => string;

Additional information

We have a pending introduce markup RFC, which is related because a registry is introduced