Fix Options to refuse allowed and subject as option names

[marcoroth]

This pull request fixes two option names being silently swallowed instead of refused. allowed and subject were keyword parameters on Options.new, so **options could collide with them and they never reached validate!.

Every other unknown option raises. These two quietly reconfigured the validator and then went missing from the JSON that reaches the native library.

LightningCSS.transform(".a {}", nonsense: true)
#=> LightningCSS::OptionError: Unknown option: nonsense

LightningCSS.transform(".a {}", subject: "x")
#=> ".a {\n}\n"

LightningCSS.transform(".a {}", allowed: [:minify])
#=> ".a {\n}\n"

Both are positional arguments now, so no option name can reach them, and the two cases above raise the same way anything else unknown does.

LightningCSS.transform(".a {}", subject: "x")
#=> LightningCSS::OptionError: Unknown option: subject

Breaking change

Options.new and Options.serialize take the options hash as their first argument.

- Options.new(minify: true)
+ Options.new({ minify: true })

- Options.serialize(options, allowed: Options::STYLE_ATTRIBUTE, subject: "a style attribute")
+ Options.serialize(options, Options::STYLE_ATTRIBUTE, "a style attribute")

LightningCSS.transform, bundle, minify and transform_style_attribute are unaffected, so this only reaches code building Options directly.

Neither name was usable as an option before this, since allowed could only narrow what KNOWN already permits and subject only changes the wording of an error. Nothing was silently dropped that a caller could have meant.