parser: parse into 0-terminated strings

This was kind of an annoying change to make since 0.11.0 has issues
where it will point to the wrong srcloc on compile errors in generic
code (which this 100% is) fortunately fixed in master. The motivation
for this change is that the arg vector already contains 0-terminated
strings, so we can avoid a lot of copies. This makes forwarding
command-line arguments to C-functions that expect zero-terminated
strings much more straightforward, and they also automatically decay
to normal slices.

Unfortunately, environment variable values are NOT zero-terminated, so
they are currently copied with zero-termination. This seems to be the
fault of Windows/WASI, both of which already are performing
allocations (Windows to convert from UTF-16 to UTF-8, and WASI to get
a copy of the environment). By duplicating the std EnvMap
implementation, we could make a version that generates 0-terminated
env vars without extra copies, but I'll skip on doing that for now.

demo/demo.zig

@@ -30,6 +30,13 @@ const cli = cmd: {
         .description = "enum choice option",
         .nice_type_name = "choice",
     });
+    cmd.stringOption(.{
+        .name = "string",
+        .short_tag = "-s",
+        .long_tag = "--string",
+        .env_var = "NOCLIP_STRING",
+        .description = "A string value option",
+    });
     cmd.addOption(.{ .OutputType = u32 }, .{
         .name = "default",
         .short_tag = "-d",
@@ -80,8 +87,8 @@ const subcommand = cmd: {
         .falsy = .{ .long_tag = "--no-flag" },
         .env_var = "NOCLIP_SUBFLAG",
     });
-    cmd.addArgument(.{ .OutputType = []const u8 }, .{ .name = "argument" });
-    cmd.addArgument(.{ .OutputType = []const u8 }, .{
+    cmd.stringArgument(.{ .name = "argument" });
+    cmd.stringArgument(.{
         .name = "arg",
         .description = "This is an argument that doesn't really do anything, but it's very important.",
     });
@@ -95,6 +102,7 @@ fn subHandler(context: []const u8, result: subcommand.Output()) !void {
 
 fn cliHandler(context: *u32, result: cli.Output()) !void {
     std.debug.print("context: {d}\n", .{context.*});
+    std.debug.print("callback is working {s}\n", .{result.string orelse "null"});
     std.debug.print("callback is working {any}\n", .{result.choice});
     std.debug.print("callback is working {d}\n", .{result.default});
     context.* += 1;