Implement vertical alignment.

BREAKING: Boxes now generate struts (CSS), which can make a line taller
than is necessary to contain its fragments.

Do not pass around unnecessary data.

Store parent text options in ResolvedBox.

This is required for vertically aligning the box with its parent.

Add options for vertical alignment.

Separate vertical alignment from horizontal positioning.

Store vertical offsets inside ProtoFragment.

Intended to make these offsets manipulable during layout.

Add internal structure for vertical offsets.

Use more specific name for the LineHeight typeclass.

Fix internal link to non-existent identifier.

Clarify that fragmentContentRect is unstandardised.

Add content rect to Fragment.

Add helper function to strut empty boxes.

Allow trimmed whitespace to generate empty fragments.

BREAKING: Output may now contain more fragments with zero glyphs,
representing runs of whitespace that was trimmed away. These runs may
also start a box at the end of a line, instead of the beginning of the
next line.

BREAKING: Trimmed whitespace now affects line height.

Prevent empty shaped runs.

This is in preparation for a breaking change that introduces some
empty fragments into the output. Since the shaped runs output is not
concerned with runs of zero glyphs, these will be proactively removed.

Add forceLeftAlign function for testing.

Generalise shiftFragment function.

Intended for testing paragraphSafeWidth.

Allow explicit control of box collapsing.

Allow generating an empty run.

Add extensible filter for invisible lines.

Store default level in BiDiLevels.

This will be required for handling empty text sequences.