allow to render "loose" tables as Nushell tables

[amtoine]

cc/ @kubouch

the following "loose" table should now render as a full table in nu_plugin_explore by default

[
    [spam, egg];

    [0, "foo"],
    [1, null],
    [2, [1, 2, 3]],
]

this can be changed with the strict_tables config.