keyboard: support ordinal modifiers

[pokey]

Some links:

• Search the code base for nextPrev. You'll need to make a new type of token called ordinal that functions identically to nextPrev, but creates an ordinalScope modifier instead of the relativeScope created by nextPrev
• Here is the shape of the ordinal modifier you'll end up creating from our keyboard grammar: https://github.com/cursorless-dev/cursorless/blob/5f94e084af3f67e07a6d6f68e478365e7a691d31/packages/common/src/types/command/PartialTargetDescriptor.types.ts#L263-L280