feat: ignore shebang lines starting with #! on first line of source

Add special handling in the lexer's nextToken function to detect a '#' character followed by '!' on the first line (currentLine == 1) and skip the entire line as a comment. If the shebang appears on any other line, or if a '#' is not followed by '!', emit a lex error for invalid character. Include test cases for valid shebang execution, shebang at EOF, shebang on a non-first line (expected error), and an invalid shebang pattern (expected error).

test/shebang/shebang.wren

@@ -0,0 +1,2 @@
+#!/bin/wren
+IO.print("ok") // expect: ok

test/shebang/shebang_at_eof.wren

@@ -0,0 +1 @@
+#!/bin/wren

test/shebang/shebang_at_other_line.wren

@@ -0,0 +1,3 @@
+// expect error line 3
+IO.print("nope")
+#!/bin/wren

test/shebang/shebang_invalid.wren

@@ -0,0 +1,3 @@
+#/invalid/shebang
+// expect error line 1
+IO.print("nope")