Honor the -w flag in REPL mode.

[csilvers]

Right now, -w (write-decls-and-statements) is only used when gomacro is called with file arguments. But it might be useful to save a record of what was run for interactive mode as well. For instance, you might want to play around with some code interactively, save the results, and then use that as the basis of writing a script.

This PR makes it so -w is respected when run as a REPL. In the existing (file-based) usage, the output of -w is based on the filename. For the new, REPL case there's nothing to base the ouptut filename on, so I just hard-code a name.

Test plan:
I ran the following:

gomacro% go run . -w
// Welcome to gomacro. Type :help for help, :copy for copyright and license.
// This is free software with ABSOLUTELY NO WARRANTY.
gomacro> func() { return 5; }
repl.go:1:8: expected 'IDENT', found '{'
gomacro> func a() { return 5; }
return: expecting 0 expressions, found 1: return 5
gomacro> a := func() int { return 5 }
gomacro> :exit

gomacro% cat repl.go
// -------------------------------------------------------------
// DO NOT EDIT! this file was generated automatically by gomacro
// Any change will be lost when the file is re-generated
// -------------------------------------------------------------

// REPL run: 2023-09-21T13:42:27-07:00
package main

func a() { return 5 }
var a = func() { return 5 }
var a = func() int { return 5 }

which shows that the declarations are being saved and emitted as expected.

[csilvers]

For the new, REPL case there's nothing to base the ouptut filename on, so I just hard-code a name.

I didn't know what the best behavior here might be -- I'm happy to do here whatever you think is best.

[csilvers]

Sorry, this was my editor auto-fixing things. I can revert if you have strong feelings.