Cannot expand `${0_1}`

[magicant]

(Issue migrated from https://osdn.net/projects/yash/ticket/39289)

I'm not sure if the paraneter name 0_1 is acceptable or not in the first place, but the current behavior is inconsistent anyway.

$ typeset 0_1=foo
$ export 0_1
$ printenv 0_1
foo
$ echo ${0_1?}
yash: parameter `0_1' is not set

Maybe there should be a way to expand a variable with an arbitrary name possibly including unusual characters. See also #28.