Treat emoji presentation sequences as fullwidth

[Jules-Bertholet]

UAX11 says:

[UTS51] emoji presentation sequences behave as though they were East Asian Wide, regardless of their assigned East_Asian_Width property value.

Lookup is done with a 2-level trie.

[Jules-Bertholet]

In terms of UTS 51 conformance, with this PR, this crate will give the correct widths for:

• The basic emoji set, including emoji presentation sequences
• The emoji keycap sequence set
• The RGI emoji flag sequence set
• The RGI emoji tag sequence set

However, it may overestimate (though never underestimate) the rendered widths of:

• The RGI emoji modifier sequence set
• The RGI emoji ZWJ sequence set

[Jules-Bertholet]

I've replaced the binary search with a better datastructure, and also added a section in the rustdoc documenting the full width rules.

[Jules-Bertholet]

The not-yet-released Unicode 16 adds 8 new non-emoji standardized variation sequences that affect width: https://unicode.org/alloc/Pipeline.html#variation_sequences, https://www.unicode.org/L2/L2023/23212r-quotes-svs-proposal.pdf. In time, we'll need to support those as well.

[Jules-Bertholet]

This PR is bloated, I'm splitting it up.