Raise on unknown browsers in targets

[marcoroth]

This pull request updates targets to raise a LightningCSS::OptionError for a browser name it doesn't know, instead of ignoring it.

A misspelled browser, like safar: 15, used to be skipped without a word, so the stylesheet was compiled as if that target wasn't there. That's the same problem LightningCSS::Options already refuses an unknown option for.

LightningCSS.transform(".a {}", targets: { safar: 15 })
#=> LightningCSS::OptionError: Unknown browser in targets: safar

LightningCSS.transform(".a {}", targets: { netscape: 4, chrome: 100, mosaic: 1 })
#=> LightningCSS::OptionError: Unknown browsers in targets: mosaic, netscape

The names are checked before any version is read, and listed in order, so the message doesn't depend on hash order. ios still reads as ios_saf.

This raises for code that passed an extra key in targets, which used to do nothing.