feat: don't completely bail if support data does not exist

I'm currently working on dynamically transpiling resources with the minimum set of transformations required by that browser, and it would be useful to not bail if a version is specified for which there is no support data. Defaulting to [either bitmask is fine](https://github.com/Rich-Harris/buble/blob/3689ddd0aaf43722ac3b94389472c8a9235fe0d3/src/index.js#L18-L19) (or perhaps by specifying a "fallback" option).

@Rich-Harris, I'm happy to open a PR just wanted to run this by you first. Let me know what you think..

Start in src/index.js around lines 18-19, where the default bitmask is defined. Trace how a browser version without support data currently causes transpilation to bail and review the surrounding option handling. Done means unsupported versions continue with an agreed fallback behavior or fallback option, with regression coverage in the relevant existing tests.

Written by the indexing model from the issue text.