[Feature] Word completion

**What problem does this solve?**
Allows word completion with human language; programmers often use completion to fill in function/variable names, so why shouldn't this be generalized to language?

**Proposed Solution**
Similar to [Simple Completion Language Server,](https://github.com/estin/simple-completion-language-server) this would offer completions to words in Harper's active dictionary.

Unlike the above example, it would conform to Harper's grammar rules, thus giving a more concise, correct list of completions.

In addition, users who want this feature don't have to install one more LSP.

**Examples**
The following:

"What is the so|"

Might offer the following completions (sourced from [WordMom](https://wordmom.com/nouns/start-so)):
- soak
- soaking
- soapbox
- soapsuds
- soapwart

etc.

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

**Additional Context**
_I'm not sure how much code needs changing to implement this, so if it's an unreasonable request, that's okay. This would improve productivity, though._

Start by reviewing Harper's core engine and the linked Simple Completion Language Server to understand how word completions could be sourced and filtered by grammar rules. Done means the engine offers concise, correct completions for an example such as "What is the so|" without requiring another LSP; the issue does not name files or tests to run.

Written by the indexing model from the issue text.