feat: Adding SingleFile appender

[nine9ths]

I'm trying to switch to logforth from log4rs and everything is looking good except for the ability to just write to a single file. My application is a short running commandline utility that can write its log to either stderr or a file. I tried using the RollingFile appender with a Rotation::Never but I have a requirement that the log file path must be able to be set explicitly and RollingFile always includes the cnt. I've gated this behind a single-file feature similar to rolling-file.

This seemed like something that could be generally useful, hence the pull request.

Thanks for taking a look!

[tisonkun]

Thanks for your contribution @nine9ths!

Generally looks good. I'll take another look later and try to merge it. Feel free to ping me if I miss it by the end of next week.

[tisonkun]

This doc test would fail because /path/to/file.log typically no permission to access and non-exist.

[nine9ths]

Oops, I always forget to make sure I've run the doc tests because I use nextest. Think it's passing now, ran with cargo test --doc --features single-file,json.

[tisonkun]

Thanks for your contribution! Merging ...

[tisonkun]

I reviewed this patch today and noticed that if the RollingFile appender can set the full file name and doesn't always include cnt, you can use it with Rotation::Never to implement the necessity.

Related to #143.

[nine9ths]

Yes, I originally tried fulfilling my use case with Rotation::Never. I chose to implement a separate struct because the RollingFile appender was already feature gated so it seemed like the style of the library to put appenders behind gates and the single file appender would be a much simpler implementation.

I'm fine with switching over to RollingFile with Never if I can control the filename. I liked the suggestion of a pluggable file namer too.

[tisonkun]

Implementing at #159

@nine9ths welcome to give a review