This commit changes the behavior on Windows:
* If $TERM is set and the `terminfo` crate is able to
successfully initialize and locate a terminfo database (this also
requires that $TERMINFO be set in the environment), then we'll
use the `TerminfoRenderer` instead of the `WindowsConsoleRenderer`
* If $TERM is set to `xterm-256color` and no terminfo database was
found, use our modern compiled-in copy (look in the `termwiz/data/`
directory for the source and compiled version of this) and use
the `TerminfoRenderer`.
* Otherwise use the `WindowsConsoleRenderer`.
In practice, this allows termwiz apps to opt in to features such as
true color support on Windows 10 build 1903 an later by setting their
`TERM=xterm-256color`. This happens to be the default behavior when
`ssh`ing in to a windows host via `wezterm`.
You can see the truecolor mode get applied by running this example:
```
cargo run --example widgets_basic --features widgets
```
with TERM set as above the background region that is painted by the app
will be a blueish/purplish color, but with it unset or set to something
invalid, it will fall back to black.
I'd like to eventually make termwiz assume the equivalent configuration
to `TERM=xterm-256color` by default on Windows 10 build 1903 and later,
but it's worth getting some feedback on how this works for clients such
as `streampager`.
cc: @quark-zju and @markbt
