Fix avoidance calculation on NO_THREADS build

[smix8]

Fixes #66793

Adds fallback for avoidance calculation on NO_THREADS builds e.g. HTML5 with no threads enabled.

[akien-mga]

Might be worth adding a comment in the documentation too, e.g.:

diff --git a/doc/classes/NavigationAgent.xml b/doc/classes/NavigationAgent.xml
index 5f8eed4619..e4b0f55dbd 100644
--- a/doc/classes/NavigationAgent.xml
+++ b/doc/classes/NavigationAgent.xml
@@ -6,6 +6,7 @@
 	<description>
 		3D agent that is used in navigation to reach a location while avoiding static and dynamic obstacles. The dynamic obstacles are avoided using RVO (Reciprocal Velocity Obstacles) collision avoidance. The agent needs navigation data to work correctly. By default this node will register to the default [World] navigation map. If this node is a child of a [Navigation] node it will register to the navigation map of the navigation node or the function [method set_navigation] can be used to set the navigation node directly. [NavigationAgent] is physics safe.
 		[b]Note:[/b] After [method set_target_location] is used it is required to use the [method get_next_location] function once every physics frame to update the internal path logic of the NavigationAgent. The returned vector position from this function should be used as the next movement position for the agent's parent Node.
+		[b]Note:[/b] By default, the expensive calculations for avoidance are done in a thread. In HTML5 exports without thread support, they will be done on the main thread, which can lead to performance issues.
 	</description>
 	<tutorials>
 	</tutorials>

(I don't know if it's the best place for it, and it should be replicated in 2D if it is.)

[smix8]

It is 2D and 3D so if added to the docs should be added for both.

EDIT
Added note to NavigationAgent, NavigationAgent2D and NavigationServer.

[akien-mga]

Thanks!

[timothyqiu]

Cherry-picked for 3.5.2