layout: Shape text only once (#31146)

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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components/layout_2020/flow/mod.rs

@@ -39,6 +39,7 @@ pub mod float;
 pub mod inline;
 mod line;
 mod root;
+pub mod text_run;
 
 pub(crate) use construct::BlockContainerBuilder;
 pub use root::{BoxTree, CanvasBackground};