Implement FT.SPELLCHECK command

The `FT.SPELLCHECK` command is used to perform spelling correction on a given query.
It helps detect misspelled words and suggest possible corrections based on custom dictionaries.

```
FT.SPELLCHECK index query [DISTANCE distance] [TERMS INCLUDE|EXCLUDE dictionary ...]
```

Related Commands

`FT.SPELLCHECK` works with custom dictionaries managed by the following commands:
- FT.DICTADD - Adds one or more terms to a dictionary.
- FT.DICTDEL - Removes terms from a dictionary.
- FT.DICTDUMP - Returns all terms stored in a dictionary.

These dictionaries act as sources for spellchecking suggestions.

Start by locating the existing FT.DICTADD, FT.DICTDEL, and FT.DICTDUMP command entry points and their dictionary handling. Compare the requested FT.SPELLCHECK syntax, distance option, and INCLUDE/EXCLUDE dictionary sources with the project's search command conventions; done means the documented command works with those options and returns spelling suggestions.

Written by the indexing model from the issue text.