Definition On Hover

**What problem does this solve?**
Brief description.

It's very useful in writing to double-check the definitions of words as you're writing, especially when you're proofreading, and swapping things out. It's crucial to make sure you're not killing meaning just for a better fit. Would be great to even have variants of this for say, hover for synonyms, and other popular usages. This should not conflict with the diagnostic hovers.

**Proposed Solution**
How should it work?
If we could implement a default hover-effect, using the same interfaces that programming languages use to give documentation snippets and API/type surface hovers, that would be the dream.

**Examples**
Show, don't tell.
I hover over show, and bring up the associated definition. In Markdown (I know dictionaries have their own format, but when viewed in a dictionary viewer they are converted into rich text, so I'm sure we can do something similar here).

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

**Additional Context**
Any other relevant info.

I would totally contribute this! This is awesome!

No files, tests, or entry points are identified. First clarify the dictionary source, rich-text representation, language coverage, and how definition hovers should coexist with diagnostic hovers; done means hovering a word shows its definition without conflicting with diagnostics.

Written by the indexing model from the issue text.