Correct "show type" feature in Source §x Typed

[martin-henz]

Currently, the deprecated type inference feature is used for the show type shortcut:

(activate the feature with control-shift-m: https://github.com/source-academy/frontend/wiki/Source-Academy-Keyboard-Shortcuts)

We should disable this for the untyped Source variants, and switch this to the declared type of names (or any if no type is declared) for the typed variants of Source.