This adds an initial implementation of font fallback, on part with the
one used in legacy layout. There are still issues. For instance, font
matching is done per unicode character rather than based on graphemes or
the shape first approach of Chrome. The idea is that these changes can
be made later.
Shape text during InlineFormattingContext construction rather than doing
it twice during fragment tree construction. This is a step on the way
toward proper font fallback.
This also moves all `TextRun` related code into `text_run.rs` to try to
trim down the size of `inline.rs`.
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