Rework lessopen implementation to use execute crate instead of run_script

[Anomalocaridid]

I replaced the lessopen implementation to use a crate called execute instead of the currently-used run_script. This has the benefit of simplifying the code a little, but more importantly fixes a few weird issues I noticed.

The issues I noticed that these changes fix are:

• batpipe not working right when using fish as a shell. Currently, bat shows the commands that batpipe prints to configure $LESSOPEN when you evaluate it in your startup config rather than the preprocessed file output.
• bat not showing any output at all when using glow to preprocess markdown files.

[Anomalocaridid]

Marking as draft because I realized my changes cause some of the integration tests to fail. Sorry for not catching it sooner.

[Anomalocaridid]

I fixed the issues with the integration tests.

However, while troubleshooting them, I came across another potential concern that I am unsure how to resolve. I noticed that unlike what I assumed in my initial implementation, less does not appear to send $LESSCLOSE's output to stdout. If I change this to match less's behavior, then that would break the integration tests that test $LESSCLOSE because they rely on bat outputting $LESSCLOSE's output. I am not sure how to change them to work around this, though.

Also, it may be worth seeing at some point if using execute instead of run_script also fixed the issue with the randomly failing integration test.