[VarDumper] add meta-data on hover
feature #12256 [VarDumper] add meta-data on hover (nicolas-grekas)

This PR was merged into the 2.6-dev branch.

Discussion
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[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

Commits
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bef48b3 [VarDumper] add meta-data on hover