Add support for loading 1D flex components from OBJ line segments.

The obj_decoder plugin is updated to parse line segments ('l') from OBJ files and represent them as degenerate triangles. The user_flexcomp compiler now supports flex dimensions of 1, using these parsed line segments to define the 1D flex elements. The compile sample's error reporting is also improved.

PiperOrigin-RevId: 928566870
Change-Id: I6920729d1b1d5aeb391401c3245ebb1d4784c190

test/user/testdata/flex_line_obj.xml

@@ -0,0 +1,5 @@
+<mujoco>
+  <worldbody>
+    <flexcomp name="line" type="mesh" file="line.obj" dim="1" radius="0.1"/>
+  </worldbody>
+</mujoco>

test/user/testdata/line.obj

@@ -0,0 +1,6 @@
+v 0 0 0
+v 2 0 0
+v 1 0 0
+v 3 0 0
+l 1 3 2
+l 2 4