Support icon attribute on resource definitions

Resolves #303. Allows the language server to recognize the `icon`
attribute supported on resources in Concourse 5.x

headless-services/concourse-language-server/src/main/java/org/springframework/ide/vscode/concourse/PipelineYmlSchema.java

@@ -236,6 +236,7 @@ public class PipelineYmlSchema implements YamlSchema {
 		addProp(t_resource, "check_every", t_duration);
 		addProp(t_resource, "tags", t_strings);
 		addProp(t_resource, "webhook_token", t_ne_string);
+		addProp(t_resource, "icon", t_ne_string);
 
 		AbstractType t_image_resource = f.ybean("ImageResource");
 		{

headless-services/concourse-language-server/src/main/resources/desc/Resource/icon.md

@@ -0,0 +1 @@
+*Optional* If specified, it must be the name of a [Material Design Icon](https://materialdesignicons.com) and will be rendered alongside the resource's name in the Concourse UI.

headless-services/concourse-language-server/src/test/java/org/springframework/ide/vscode/concourse/ConcourseEditorTest.java

@@ -1591,6 +1591,7 @@ public class ConcourseEditorTest {
 				"resources:\n" +
 				"- name: sts4\n" +
 				"  type: git\n" +
+				"  icon: foo\n" + 
 				"  check_every: 5m\n" +
 				"  webhook_token: bladayadayaaa\n" +
 				"  source:\n" +
@@ -1599,6 +1600,7 @@ public class ConcourseEditorTest {
 
 		editor.assertHoverContains("name", "The name of the resource");
 		editor.assertHoverContains("type", "The type of the resource. Each worker advertises");
+  	editor.assertHoverContains("icon", "name of a [Material Design Icon]");
 		editor.assertHoverContains("source", 2, "The location of the resource");
 		editor.assertHoverContains("webhook_token", "web hooks can be sent to trigger an immediate *check* of the resource");
 		editor.assertHoverContains("check_every", "The interval on which to check for new versions");
@@ -3509,6 +3511,7 @@ public class ConcourseEditorTest {
 		editor.assertCompletionLabels(
 				//For the 'exact' context:
 				"check_every",
+				"icon",
 				"tags",
 				"webhook_token",
 				//"name", exists