add itv_closureE, itv_interiorE, and helper lemmas

[t6s]

Motivation for this change

Reviewing PR #1674 , it was noticed that there is not much theory
connecting closure/interior operators and set-intervals.
This PR adds lemmas to amend that, especially itv_interiorE and itv_closureE
stating that the interior/closure of an interval is an open/closed interval respectively,
for intervals in realFieldType.

This should resolve the following comment:
https://github.com/math-comp/analysis/pull/1674/changes/BASE..34697b2642259157cd3678e64092864dcd0ca4ef#r2685444293

Checklist

• added corresponding entries in CHANGELOG_UNRELEASED.md

• added corresponding documentation in the headers

Reference: How to document

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