Support `exhibitfiles` to set files that need to be "exhibited" in the current workspace; fix a typo

[myhsia]

Support exhibitfiles to set files that need to be "exhibited" in the current workspace

Some users merge a README.md file, for example, into a *.dtx file. Once they change the markdown file, they should modify the source *.dtx file.

But l3build won't move the generated (unpacked) markdown file to the main directory (corresponding to currentdir in l3build-variables), and users usually need to keep the markdown file (or other files) in the main directory so that it can be displayed on the GitHub repository homepage.

So, a variable exhibitfiles is added: after unpack is executed, the enumerated text files will be copied from unpackdir to currentdir; and certainly, these exhibitfiles will be cleaned after l3build clean.

Fix a typo

In l3build.dtx, line 2249: \loggingoutput is misspelled in the macro environment.