chore: add a path-specific github instructions related to API guidelines

Author: Adel Khamatov

.github/instructions/API_conventions.instructions.md

@@ -0,0 +1,20 @@
+---
+applyTo: "**/packages/devextreme/js/common.d.ts"
+---
+
+When helping me write code or reviewing my existing code, please follow these specific code style conventions:
+
+1. **Acronym Casing in Identifiers:**
+   - Treat all acronyms as regular words in camelCase and PascalCase
+   - Capitalize only the first letter of each acronym, not the entire acronym
+   - Examples:
+     - ✓ `testDom` (not `testDOM`)
+     - ✓ `parseHtml` (not `parseHTML`)
+     - ✓ `createJsonParser` (not `createJSONParser`)
+     - ✓ `apiEndpoint` (not `APIEndpoint`)
+   - This applies to all acronyms regardless of length (DOM, HTML, JSON, API, etc.)
+
+2. **JSDoc Format - Omit @type:**
+   - Do not include the `@type` tag in JSDoc comments when the type is already indicated by TypeScript type annotation
+
+Consistently apply these conventions to all code you review, suggest, complete.