This patch allows running `alacritty migrate` to automatically apply
configuration changes made to the TOML format, like moving `ipc_socket`
to `general.ipc_socket`.

This should reduce the friction of moving around individual options
significantly, while also persisting the format of the existing TOML
file thanks to `toml_edit`.

The YAML migration has been simplified significantly to only switch the
format of the file from YAML to TOML. The new TOML features are used for
everything else.

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.