fmt: allow parsing files without `fn main()` (#5681)

vlib/v/fmt/tests/no_main_expected.vv

@@ -0,0 +1 @@
+println('hello world')

vlib/v/fmt/tests/no_main_input.vv

@@ -0,0 +1 @@
+println( "hello world" )

vlib/v/parser/parser.v

@@ -452,6 +452,8 @@ pub fn (mut p Parser) top_stmt() ast.Stmt {
 					file: p.file_name
 					return_type: table.void_type
 				}
+			} else if p.pref.is_fmt {
+				return p.stmt(false)
 			} else {
 				p.error('bad top level statement ' + p.tok.str())
 				return ast.Stmt{}