Add navigation path simplification

[smix8]

Adds navigation path simplification for NavigationServer and NavigationAgent.

Resolves #60277
Resolves #19011 (the oldest navigation issue back from 2018 ... weeee!)

This adds a variant of Ramer-Douglas-Peucker algorithm for 2D and 3D to simplify navigation paths.

With this simplification enabled and a higher epsilon all the path points that are not critical points (like a corner or steep ground obstacle) will be removed from the path. The epsilon is in world units, so meters for 3D and pixels for 2D.

This helps with all kinds of agent movement problems in "open fields" and also helps with "steering" agents because they only have more critical path points to aim for.

Note that this simplification is a post-processing of the path query, so it does not help with path query performance, it adds performance cost.

NavigationAgents

The NavigationAgent2D and NavigationAgent3D have 2 new properties added to enable the simplification and control the epsilon.

NavigationPathQueryParameters

The NavigationPathQueryParameters2D and NavigationPathQueryParameters3D also have these 2 new properties added.

var parameters: NavigationPathQueryParameters2D = NavigationPathQueryParameters2D.new()
parameters.simplify_path = true
parameters.simplify_epsilon = 2.0

var parameters: NavigationPathQueryParameters3D = NavigationPathQueryParameters3D.new()
parameters.simplify_path = true
parameters.simplify_epsilon = 0.01

NavigationServer

The NavigationServer2D and NavigationServer3D have a new simplify_path() function added.

var path: PackedVector2Array = ....
var epsilon: float = 2.0
var simplified_path: PackedVector2Array = NavigationServer2D.simplify_path(path, epsilon)

var path: PackedVector3Array = ....
var epsilon: float = 0.01
var simplified_path: PackedVector3Array = NavigationServer3D.simplify_path(path, epsilon)

Because this NavigationServer function is not specific to navigation it can also be used with "any" path arrays.

E.g. it can also be used with point arrays commonly used in other nodes and classes like Line2D, Path2D, or Path3D.

[mhilbrunner]

Approving the docs. The feature is useful to me, and the implementation looks clean and self-contained, and I didn't spot anything wrong or surprising during a brief code review. I however didn't build and test this.

[fire]

Approving the feature, did not test the code yet.

[akien-mga]

CC @Scony

[Scony]

Looks good - just an optional suggestion - I'd extend descriptions in .xml files so that "reason why this useful" is added. At the moment descriptions like:

If [code]true[/code] a simplified version of the path will be returned with less critical path points removed. The simplification amount is controlled by [member simplify_epsilon].

just explain what it does. Some users may find it useful if there's a one sentence summarizing when it's worth enabling that path optimization.

[akien-mga]

Thanks!