feat(Geometry/Euclidean): theorems to prove line parallel from relation between oangles

[benpigchu]

We have EuclideanGeometry.two_zsmul_oangle_of_parallel to derive relationship of oangles from parallel lines, but we do not have theorems to prove parallel of lines from relationship of oangles. This PR add theorems for that, and their iff version.

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[github-actions]

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PR summary e676b4c63b

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

---

Declarations diff (regex)

+ parallel_of_two_zsmul_oangle
+ span_eq_of_two_zsmul_oangle_eq_zero
+ span_eq_of_two_zsmul_oangle_left
+ span_eq_of_two_zsmul_oangle_right
+ two_zsmul_oangle_eq_zero_iff_span_eq
+ two_zsmul_oangle_iff_parallel
+ two_zsmul_oangle_iff_vectorSpan_eq
+ two_zsmul_oangle_left_iff_span_eq
+ two_zsmul_oangle_of_span_eq
+ two_zsmul_oangle_right_iff_span_eq
+ vectorSpan_eq_of_two_zsmul_oangle

You can run this locally as follows

## from your `mathlib4` directory:
git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci

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

## more verbose report:
../mathlib-ci/scripts/pr_summary/declarations_diff.sh long <optional_commit>

The doc-module for scripts/pr_summary/declarations_diff.sh in the mathlib-ci repository contains some details about this script.

Declarations diff (Lean)

✅ Lean-aware diff — post-build, computed from the Lean environment (commit e676b4c).

• +11 new declarations
• −0 removed declarations

+EuclideanGeometry.parallel_of_two_zsmul_oangle
+EuclideanGeometry.two_zsmul_oangle_iff_parallel
+EuclideanGeometry.two_zsmul_oangle_iff_vectorSpan_eq
+EuclideanGeometry.vectorSpan_eq_of_two_zsmul_oangle
+Orientation.span_eq_of_two_zsmul_oangle_eq_zero
+Orientation.span_eq_of_two_zsmul_oangle_left
+Orientation.span_eq_of_two_zsmul_oangle_right
+Orientation.two_zsmul_oangle_eq_zero_iff_span_eq
+Orientation.two_zsmul_oangle_left_iff_span_eq
+Orientation.two_zsmul_oangle_of_span_eq
+Orientation.two_zsmul_oangle_right_iff_span_eq

---

No changes to strong technical debt.
No changes to weak technical debt.

Current commit e676b4c63b
Reference commit dec5b2b780

This script lives in the mathlib-ci repository. To run it locally, from your mathlib4 directory:

git clone https://github.com/leanprover-community/mathlib-ci.git ../mathlib-ci
../mathlib-ci/scripts/reporting/technical-debt-metrics.py 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).