This aligns Babel's behavior with microsoft/TypeScript#26313 for class fields. Specifically:

if (isConstOrReadonly && !node.type) {
    if (isInvalidInitializer) {
        return grammarErrorOnNode(node.initializer!, Diagnostics.A_const_initializer_in_an_ambient_context_must_be_a_string_or_numeric_literal);
    }
}
else {
    return grammarErrorOnNode(node.initializer!, Diagnostics.Initializers_are_not_allowed_in_ambient_contexts);
}
if (!isConstOrReadonly || isInvalidInitializer) {
    return grammarErrorOnNode(node.initializer!, Diagnostics.Initializers_are_not_allowed_in_ambient_contexts);
}

However, this PR does not attempt to mimic the isInvalidInitializer part, which checks if the initializer is a string/number literal or an enum reference. Doing so "correctly" would likely require type information. Quoting evanw/esbuild@147163e,

To future-proof this in case TypeScript allows more expressions as initializers in the future (such as null), esbuild will allow any expression as an initializer and will leave the specifics of TypeScript's special-casing here to the TypeScript type checker.

Instead, we avoid all errors with declare readonly that don't have a type annotation.