This fixes a regression introduced in bd49067 which broke the override
of configuration file variables using `alacritty msg config`.

To fix this the `replace` functionality was rewritten to behave more
like the `serde_utils::merge` where entire values are inserted into the
existing structure rather than separating the keys from the values.

Fixes: bd49067 (Switch to TOML configuration format)

This switches Alacritty's default configuration format from yaml to
toml. While yaml is still supported, it is done by converting it to toml
and should be removed entirely in the future.

All existing features were persisted based on my testing. Behavior
should not change much, though `--option` might have slightly different
behavior since the entire line is not interpreted as one line of toml.

A new `alacritty migrate` subcommand has been added which allows
automatic migration from yaml to toml. This also could be used as a
facility to automatically fix configuration file changes in the future.

Closes #6592.

Fixes #6879.
Fixes #6874.

Fixes #6644.
Fixes #6615.
Fixes #6558.
Fixes #6515.
Fixes #3187.
Fixes #62.

This patch adds a new mechanism for changing configuration options
without editing the configuration file, by sending options to running
instances through `alacritty msg`.

Each window will load Alacritty's configuration file by default and then
accept IPC messages for config updates using the `alacritty msg config`
subcommand. By default all windows will be updated, individual windows
can be addressed using `alacritty msg config --window-id
"$ALACRITTY_WINDOW_ID"`.

Each option will replace the config's current value and cannot be reset
until Alacritty is restarted or the option is overwritten with a new
value.

Configuration options are passed in the format `field.subfield=value`,
where `value` is interpreted as yaml.

Closes #472.

Fixes #6239.
Fixes #5975.
Fixes #5876.
Fixes #5767.
Fixes #4484.
Fixes #3139.

Remove the `font.use_thin_strokes` config, which only did anything on
macOS and only prior to Big Sur. Instead, we will enable or disable
"font smoothing" on macOS based on the `AppleFontSmoothing` user
default.

These changes let users get the "thin strokes" behavior by setting
`AppleFontSmoothing` to 0 with:

```sh
$ defaults write -g AppleFontSmoothing -int 0
```

(Or replace `-g` with `org.alacritty` to apply this setting only to
Alacritty.app, rather than the whole system.)

Add a `removed` config attribute to show helpful warnings to users
who are using config options that don't do anything anymore, and apply
this attribute to `font.use_thin_strokes`.

Bump `crossfont` to 0.5.0 to pick up the new font smoothing behavior.
This release also includes a fix for a crash when trying to load a
disabled font.

Fixes #4616.
Fixes #6108.