Allow question marks inside of plain scalars in flow collections

[perlpunk]

See also yaml/libyaml#105

http://yaml.org/spec/1.1/#id907281
Question marks aren't mentioned as special inside of plain scalars here.
To be an indicator for an explicit key, it must be at the beginning and followed by a space or newline.

The following cases should be allowed:

[foo?bar]
[foo ? bar]

This would allow [ http://example/query?a=b ].

Even this one should be allowed, but it would potentially break existing code:

[?foo]
# same as
["?foo"]
# but currently parsed as
[? foo ]

The Spec:

The first plain character is further restricted to avoid most indicators as these
would cause ambiguity with various YAML structures. However, the first character
may be “-”, “?” or “:” provided it is followed by a non-space character.

[157] ns-plain-first-char(c) ::= ( ns-plain-char(c) - c-indicator )
                                 | ( ( “-” | “?” | “:” ) ns-plain-char(c) )

[ingydotnet]

We should probably allow all of https://play.yaml.io/main/parser?input=LSBbZm9vP2Jhcl0KLSBbZm9vID8gYmFyXQotIFs_Zm9vIGJhcl0=
in libyaml and pyyaml

Even the breaking change as I can't see anyone actually depending on libyaml the way it currently is for [?foo]