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 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.