This commit adds a few basic variable substitution operations:
- length
Find the length of a string or a list
- length_across
Find the lengths of things inside a list
- remove_{suffix,prefix}
Remove a suffix or a prefix from all the passed values
- regex_replace
Replace all matches of a given regex with a given template
- split
Split the given string with the given delimiter (or to its
code points if the delimiter is empty)
- concat_lists
concatenates any given lists into one
Closes#4316 (the ancient version of this same feature)
There's no guarantee that the last executed command will have a zero
exit code, and so the shell exit code may or may not be zero, even if
all the tests pass.
Also changes the `test || echo fail && exit` to
`if not test { echo fail && exit }`, since that's nicer-looking.
