feat: Add decidable membership for Interval

[kingiler]

Implemented membership and corresponding decidable instance for Interval.

Related #mathlib4 > Proposal: Add decidable membership for Interval.

[github-actions]

PR summary 51192f7a45

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

---

Declarations diff

+ instance (priority := 100) : Membership α (Interval α)
+ instance [DecidableLE α] (a : α) (s : Interval α) : Decidable (a ∈ s)
+ instance [DecidableLE α] : Decidable (a ∈ s)

You can run this locally as follows

## summary with just the declaration names:
./scripts/declarations_diff.sh <optional_commit>

## more verbose report:
./scripts/declarations_diff.sh long <optional_commit>

The doc-module for script/declarations_diff.sh contains some details about this script.

---

No changes to technical debt.

You can run this locally as

./scripts/technical-debt-metrics.sh pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).

[eric-wieser]

Please make sure to include a link to the Zulip discussion when one exists, to prevent two entirely separate discussions occurring.

[kingiler]

I have added zulip link of the discussion.