Provide support for dynamically loading font data

[dpvc]

This PR adds the ability to specify font extensions that will be loaded dynamically when less-used characters are needed. You can specify any number of extensions, and these can contain characters from any number of variants, or additional delimiters. The specification for a dynamic extension includes the file to load for the extension, along with the ranges of characters that it provides in each variant that it modifies. When an output jax needs one of these characters or delimiters, the extension is loaded and set up, and the expression is reprocessed.