chore: add copilot instructions

Author: mdevils

.github/copilot-instructions.md

@@ -0,0 +1,21 @@
+This is a CSS selector parser which tries to follow latest CSS standards and also support experimental features.
+
+Project structure:
+- `src/` contains all the source code.
+- `src/ast.ts` Abstract Syntax Tree.
+- `src/index.ts` should contain all the public API and types used in public API.
+- `src/indexes.ts` indexes for faster lookup.
+- `src/parser.ts` contains the parser itself which produces AST from CSS selector string.
+- `src/pseudo-signatures.ts` contains types and helper function for pseudo-classes and pseudo-elements.
+- `src/renderer.ts` contains the renderer which converts AST back to CSS selector string.
+- `src/syntax-definitions.ts` contains types and implementations of syntax definitions for different CSS versions.
+- `src/utils.ts` helper functions.
+
+We always write TypeScript with single quotes and 4 spaces for indentation,
+so when your responses include TypeScript code, please follow those conventions.
+
+All the imports include ".js" file extension because of the new ES module resolution algorithm.
+
+We write code for better performance, but not at the cost of readability.
+
+We write self-explanatory variable names and comments in English.