[#1711] Support singleton types

Problem: in LIGO some builtin types have a singleton type marker
(for e.g. `sapling_state` and `sapling_transaction`). However, in
`ligo-squirrel` in `Cli.Json` they are not supported. In the variables
pane they'll look like `(* unsupported type Singleton *)`.

Solution: copypaste `literal_value` from LIGO compiler into `Cli.Json`
and add a conversion of `LigoTypeLiteralValue` to `LIGO Info`.