add Dynamic Imports and BigInt as ES2020

es2020.md

@@ -0,0 +1,43 @@
+This document specifies the extensions to the core ESTree AST types to support the ES2020 grammar.
+
+# Literal
+
+```js
+extend interface Literal <: Expression {
+    type: "Literal";
+    value: string | boolean | null | number | RegExp | bigint;
+}
+```
+
+- `value` property can be a `BigInt` value to represent `BigInt` literals
+  such as `100n`.
+
+## BigIntLiteral
+
+```js
+interface BigIntLiteral <: Literal {
+  bigint: string;
+}
+```
+
+- `bigint` property is the string representation of the `BigInt` value.
+  It doesn't include the suffix `n`.
+- In environments that don't support `BigInt` values, `value` property will be
+  `null` as the `BigInt` value can't be represented natively.
+
+# Expressions
+
+## ImportExpression
+
+```js
+interface ImportExpression <: Expression {
+  type: "ImportExpression";
+  source: Expression;
+}
+```
+
+- `ImportExpression` node represents Dynamic Imports such as `import(source)`.
+  The `source` property is the importing source as similar to [ImportExpression]
+  node, but it can be an arbitrary expression node.
+
+[ImportExpression]: es2015.md#importdeclaration