Add radio inputs

[bradgessler]

Related to issue #13, this PR will add support for radio buttons to Superform.

[nimmolo]

@bradgessler is there a blocker here...or can I help?

[bradgessler]

My only blocker is I don't have enough time, so yep, help is very welcome.

[nimmolo]

Ok. Should I start a new PR or pick up on #33, i.e. should multiple select go first?

Following from the discussion on #33, it seems like a radio field PR could merge separately from the multiple select PR, since a radio cannot accept mutliple values by definition. I'm interested in getting #33 working too, but... one thing at a time.

[bradgessler]

Yeah knocking out #33 would be fantastic, then extend that with radio buttons. 👍

[nimmolo]

PR added. One note: I don't think the radio buttons will need to extend the multiple functionality, since radios do not accept multiple input by definition.

I'm noticing that the README is inconsistent regarding the API, it would help my buddy Claude if we can clean that up. Many places have Field but it's apparently field.

field(field_name).select
Field(field_name).select

I'm going to add README corrections to my version of PR 33.

Also that author brought up a good point, the spec.required_ruby_version = "2.6.0" but the codebase of Superform uses Ruby 2.7 syntax (argument forwarding) throughout. I'll change the version in the superform.gemspec to 2.7.0