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.

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.

Previously Alacritty was using two different ways to reference lines in
the terminal. Either a `usize`, or a `Line(usize)`. These indexing
systems both served different purposes, but made it difficult to reason
about logic involving these systems because of its inconsistency.

To resolve this issue, a single new `Line(i32)` type has been
introduced.  All existing references to lines and points now rely on
this definition of a line.

The indexing starts at the top of the terminal region with the line 0,
which matches the line 1 used by escape sequences. Each line in the
history becomes increasingly negative and the bottommost line is equal
to the number of visible lines minus one.

Having a system which goes into the negatives allows following the
escape sequence's indexing system closely, while at the same time making
it trivial to implement `Ord` for points.

The Alacritty UI crate is the only place which has a different indexing
system, since rendering and input puts the zero line at the top of the
viewport, rather than the top of the terminal region.

All instances which refer to a number of lines/columns instead of just a
single Line/Column have also been changed to use a `usize` instead. This
way a Line/Column will always refer to a specific place in the grid and
no confusion is created by having a count of lines as a possible index
into the grid storage.