[VarDumper] add meta-data on hover

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

This PR adds meta-data on hover to HTML dumps:
- strings length
- constants value
- control chars code
- local refs count