Feature: Allow words containing dashes (`-`) and spaces (` `) in the custom dictionary.

**What problem does this solve?**
Currently, in order to have e.g. the name of a place that contains dashes in its name, you have to put each of its pieces in the custom dictionary.

This is also applicable for spaces, like in the case of `Messerschmitt 109`.

**Proposed Solution**
1. Entries in Harper's dictionary that contain dashes/spaces should be protected against `SpellCheck`, just like ones without.
2. Those words should be suggested to use dashes if dashes are used, and spaces if spaces are used.

**Examples**
`.harper-dictionary.txt` - without this feature:
```
Bernières
sur
Mer
Messerschmitt
109
```

With this feature:
```
Bernières-sur-Mer
Messerschmitt 109
```

**Component**
- [x] Core engine
- [ ] Plugin/Extension
- [ ] Other: _____

Start in the core engine by tracing how entries from `.harper-dictionary.txt` are loaded and how `SpellCheck` handles dictionary words. Verify how hyphenated and space-separated entries are protected and how suggestions preserve their separators, using the existing core-engine behavior as the completion check.

Written by the indexing model from the issue text.