feat(core): Added Path.clean() method. Closes #3038 and #3056

This is a fix for bug #3038 which was investigated by @BenJamesBen
who also proposed a fix in PR #3056

However, after discussing the matter, we agreed it would be better
to have a generic method in core to guard against the issue of
spurious drawing operations.

This commit adds the `Path.clean()` method that does exactly that,
as well as its documentation.

markdown/dev/reference/api/path/clean/en.md

@@ -0,0 +1,61 @@
+---
+title: Path.clean()
+---
+
+The `Path.clean()` method removes spurious drawing operations from a path.
+
+A _spurious_ drawing operation is one that has no effect, but can still cause
+problems if left in place.  For example, a line from a given point to the same
+given point will not cause any problems as such, but can trip up things like
+path offset and other methods. For this reason, such drawing operations can be
+cleaned up with the `Path.clean()` method.
+
+As this method is called under the hood to guard against various scenarios
+where spurious segments could cause an issue, you should have no need to call
+this method yourself explicitly, but it's there if you need it.  path that you
+pass it.
+
+## Signature
+
+```js
+Path path.clean()
+```
+
+## Example
+
+<Example caption="Example of the Path.clean() method">
+```js
+({ Point, points, Path, paths, snippets, Snippet, part }) => {
+
+  points.A = new Point(10, 10)
+  points.B = new Point(10, 20)
+  points.C = new Point(10, 30)
+  points.D = new Point(90, 10)
+  points.E = new Point(90, 20)
+  points.F = new Point(90, 30)
+  
+  paths.a = new Path()
+    .move(points.A)
+    .line(points.C)
+    .line(points.B)
+    .line(points.B) // spurious op
+    .line(points.E)
+    .line(points.F)
+    .curve_(points.F, points.F) // another spurious op
+    .line(points.D)
+    .addClass('lining')
+
+  paths.b = paths.a
+    .clone()
+    .clean()
+    .addClass('interfacing')
+
+  paths.a.addText(`${paths.a.ops.length} ops in a`, 'center fill-lining')
+  paths.b.addText(`${paths.b.ops.length} ops in b`, 'center fill-note')
+    .attr('data-text-dy', 7)
+
+  return part
+}
+```
+</Example>
+