Update meta model for diagnostic messages and support markup content (#2260)

Update meta model

Author: dbaeumer

_specifications/lsp/3.18/metaModel/metaModel.json

@@ -9159,10 +9159,20 @@
 				{
 					"name": "message",
 					"type": {
-						"kind": "base",
-						"name": "string"
+						"kind": "or",
+						"items": [
+							{
+								"kind": "base",
+								"name": "string"
+							},
+							{
+								"kind": "reference",
+								"name": "MarkupContent"
+							}
+						]
 					},
-					"documentation": "The diagnostic's message. It usually appears in the user interface"
+					"documentation": "The diagnostic's message. It usually appears in the user interface.\n\n@since 3.18.0 - support for MarkupContent. This is guarded by the client\ncapability `textDocument.diagnostic.markupMessageSupport`.",
+					"since": "3.18.0 - support for MarkupContent. This is guarded by the client\ncapability `textDocument.diagnostic.markupMessageSupport`."
 				},
 				{
 					"name": "tags",
@@ -13384,6 +13394,17 @@
 					},
 					"optional": true,
 					"documentation": "Whether the clients supports related documents for document diagnostic pulls."
+				},
+				{
+					"name": "markupMessageSupport",
+					"type": {
+						"kind": "base",
+						"name": "boolean"
+					},
+					"optional": true,
+					"documentation": "Whether the client supports `MarkupContent` in diagnostic messages.\n\n@since 3.18.0\n@proposed",
+					"since": "3.18.0",
+					"proposed": true
 				}
 			],
 			"extends": [

_specifications/lsp/3.18/metaModel/metaModel.schema.json

@@ -3,7 +3,7 @@
   "definitions": {
     "AndType": {
       "additionalProperties": false,
-      "description": "Represents an `and`type (e.g. TextDocumentParams & WorkDoneProgressParams`).",
+      "description": "Represents an `and` type (e.g. TextDocumentParams & WorkDoneProgressParams`).",
       "properties": {
         "items": {
           "items": {