LibJS: Implement the 'Hashbang Grammar for JS' proposal

Stage 3 since August 2019 - we already have shebang stripping
implemented in js(1), so this removes it from there in favor of adding
support to the lexer directly.

Most straightforward proposal and implementation I've ever seen :^)

https://github.com/tc39/proposal-hashbang

Userland/Libraries/LibJS/Lexer.cpp

@@ -308,7 +308,9 @@ bool Lexer::is_line_comment_start(bool line_has_token_yet) const
         // "-->" is considered a line comment start if the current line is only whitespace and/or
         // other block comment(s); or in other words: the current line does not have a token or
         // ongoing line comment yet
-        || (match('-', '-', '>') && !line_has_token_yet);
+        || (match('-', '-', '>') && !line_has_token_yet)
+        // https://tc39.es/proposal-hashbang/out.html#sec-updated-syntax
+        || (match('#', '!') && m_position == 1);
 }
 
 bool Lexer::is_block_comment_start() const

Userland/Libraries/LibJS/Tests/comments-basic.js

@@ -33,3 +33,12 @@ test("unterminated multi-line comment", () => {
     expect("/* foo").not.toEval();
     expect("foo /*").not.toEval();
 });
+
+test("hashbang comments", () => {
+    expect("#!").toEvalTo(undefined);
+    expect("#!/bin/js").toEvalTo(undefined);
+    expect("#!\n1").toEvalTo(1);
+    expect(" #!").not.toEval();
+    expect("\n#!").not.toEval();
+    expect("#!\n#!").not.toEval();
+});