Sorgenfrey line and properties

[motikaku]

fixes #1044

Motivation for this change

This PR demonstrates separation axioms. It defines the Sorgenfrey line and proves it totally disconnected. We intend to also prove that it is T6. This topology is mentioned by @zstone1 in the discussion below.

https://github.com/math-comp/analysis/pull/677#pullrequestreview-1133697680

Coauthored-by @garrigue @t6s .

Checklist

• [ ] added corresponding entries in CHANGELOG_UNRELEASED.md

• [ ] added corresponding documentation in the headers

Reference: How to document

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[garrigue]

BTW, T6 is mentioned in the header of separation_axioms.v, but there does not seem to be a definition for it.