feat: multi-word dictionary entries

There are some cases where you want a spelling to be valid only in a certain context, for instance a title in a foreign language.

For instance, one document I have contains the term Dao De Ching, but it is considered a misspelling. I want these words only to be considered correct when they occur in the complete title. So, for instance De Ching would be considered incorrect, but Dao De Ching would be correct.

I tried adding the complete title to the file dictionary, but this didn't work.

The issue names no files, tests, or entry points, so first locate the dictionary handling and spelling validation paths in the Rust project. Investigate how dictionary entries are parsed and matched, then define behavior where “Dao De Ching” is accepted as a complete phrase while “De Ching” remains incorrect; done means this behavior is covered by tests.

Written by the indexing model from the issue text.