Fixes #8849

Previously, the `parseAutoStruct` function that was used to parse
generic structs for the config simply split the input value on commas
without taking into account quoting or escapes. This led to problems
because it was impossible to include a comma in the value of config
entries that were parsed by `parseAutoStruct`. This is particularly
problematic because `ghostty +show-config --default` would produce
output like the following:

```
command-palette-entry = title:Focus Split: Next,description:Focus the next split, if any.,action:goto_split:next
```

Because the `description` contains a comma, Ghostty is unable to
parse this correctly. The value would be split into four parts:

```
title:Focus Split: Next
description:Focus the next split
 if any.
action:goto_split:next
```

Instead of three parts:

```
title:Focus Split: Next
description:Focus the next split, if any.
action:goto_split:next
```

Because `parseAutoStruct` simply looked for commas to split on, no
amount of quoting or escaping would allow that to be parsed correctly.

This is fixed by (1) introducing a parser that will split the input
to `parseAutoStruct` into fields while taking into account quotes and
escaping. And (2) changing the `ghostty +show-config` output to put the
values in `command-palette-entry` into quotes so that Ghostty can parse
it's own output.

`parseAutoStruct` will also now parse double quoted values as a Zig
string literal. This makes it easier to embed control codes, whitespace,
and commas in values.

Detecting the launch source frequently failed because various launchers
fail to sanitize the environment variables that Ghostty used to
detect the launch source. For example, if your desktop environment was
launched by `systemd`, but your desktop environment did not sanitize the
`INVOCATION_ID` or the `JOURNAL_STREAM` environment variables, Ghostty
would assume that it had been launched by `systemd` and behave as such.

This led to complaints about Ghostty not creating new windows when users
expected that it would.

To remedy this, Ghostty no longer does any detection of the launch
source. If your launch source is something other than the CLI, it must
be explicitly speciflied on the CLI. All of Ghostty's default desktop
and service files do this. Users or packagers that create custom desktop
or service files will need to take this into account.

On GTK, the `desktop` setting for `gtk-single-instance` is replaced with
`detect`. `detect` behaves as `gtk-single-instance=true` if one of the
following conditions is true:

1. If no CLI arguments have been set.
2. If `--launched-from` has been set to `desktop`, `dbus`, or `systemd`.

Otherwise `detect` behaves as `gtk-single-instance=false`.

We don't really have any large outstanding regressions on -ng to warrant
keeping this alive anymore. ¡Adiós!