[node/readline] expose line/cursor properties + tests (#40513)

The current "line" and "cursor" states are often necessary when developing applications that use readline for prompts.

See https://github.com/nodejs/node/issues/30347

types/node/readline.d.ts

@@ -13,6 +13,13 @@ declare module "readline" {
     class Interface extends events.EventEmitter {
         readonly terminal: boolean;
 
+        // Need direct access to line/cursor data, for use in external processes
+        // see: https://github.com/nodejs/node/issues/30347
+        /** The current input data */
+        readonly line: string;
+        /** The current cursor position in the input line */
+        readonly cursor: number;
+
         /**
          * NOTE: According to the documentation:
          *

types/node/test/readline.ts

@@ -71,6 +71,17 @@ const rl: readline.ReadLine = readline.createInterface(new stream.Readable());
     rl.write('asd', key);
 }
 
+{
+    const data: string | Buffer = "test";
+    rl.line; // $ExpectType string
+    rl.cursor; // $ExpectType number
+
+    rl.write(data);
+
+    rl.line; // $ExpectType string
+    rl.cursor; // $ExpectType number
+}
+
 {
     const strm: NodeJS.WritableStream = new stream.Writable();
     const x = 1;

types/node/v10/readline.d.ts

@@ -13,6 +13,13 @@ declare module "readline" {
     class Interface extends events.EventEmitter {
         readonly terminal: boolean;
 
+        // Need direct access to line/cursor data, for use in external processes
+        // see: https://github.com/nodejs/node/issues/30347
+        /** The current input data */
+        readonly line: string;
+        /** The current cursor position in the input line */
+        readonly cursor: number;
+
         /**
          * NOTE: According to the documentation:
          *