Handle out-of-range values in Dense ZeroTrie

[sffc]

Currently we panic when trying to build a dense ZeroTrie with values that don't fit in the dense matrix. What we should do instead:

1. Select the row offset that gets the greatest number of values into the dense matrix.
2. Put all out-of-range values into the primary trie, as we do with sparse rows.

Docs for background: https://unicode-org.github.io/icu4x/rustdoc/zerotrie/dense/struct.ZeroAsciiDenseSparse2dTrieOwned.html

[Cdog1526]

Hey, could I try to handle this?