feat(ls): add lint rule for OpenAPI 3.x.y Schema Object xml field (#3710)

Author: char0n

packages/apidom-ls/src/config/codes.ts

@@ -68,6 +68,7 @@ enum ApilintCodes {
   SCHEMA_NULLABLE_NOT_RECOMMENDED,
   SCHEMA_MISSING_CORE_FIELDS,
   SCHEMA_NULLABLE,
+  SCHEMA_XML,
 
   DUPLICATE_KEYS = 14999,
   NOT_ALLOWED_FIELDS = 15000,

packages/apidom-ls/src/config/common/schema/lint/index.ts

@@ -27,6 +27,7 @@ import exclusiveMaximumPatternLint from './exclusive-maximum--pattern';
 import exclusiveMaximumTypeLint from './exclusive-maximum--type';
 import exclusiveMinimumPatternLint from './exclusive-minimum--pattern';
 import exclusiveMinimumTypeLint from './exclusive-minimum--type';
+import xmlTypeLint from './xml--type';
 import externalDocsTypeLint from './external-docs--type';
 import formatTypeLint from './format--type';
 import ifNonThenLint from './if--non-then';
@@ -111,6 +112,7 @@ const schemaLints = [
   exclusiveMaximumTypeLint,
   exclusiveMinimumPatternLint,
   exclusiveMinimumTypeLint,
+  xmlTypeLint,
   externalDocsTypeLint,
   formatTypeLint,
   ifNonThenLint,

packages/apidom-ls/src/config/common/schema/lint/xml--type.ts

@@ -0,0 +1,20 @@
+import { DiagnosticSeverity } from 'vscode-languageserver-types';
+
+import ApilintCodes from '../../../codes';
+import { LinterMeta } from '../../../../apidom-language-types';
+import { OpenAPI3 } from '../../../openapi/target-specs';
+
+const xmlTypeLint: LinterMeta = {
+  code: ApilintCodes.SCHEMA_XML,
+  source: 'apilint',
+  message: 'xml must be an object',
+  severity: DiagnosticSeverity.Error,
+  linterFunction: 'apilintElementOrClass',
+  linterParams: ['xml'],
+  marker: 'value',
+  target: 'xml',
+  data: {},
+  targetSpecs: OpenAPI3,
+};
+
+export default xmlTypeLint;