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)
Non-interactive shells (i.e. when running scripts) do not need this
functionality, so they are a boatload of wasted time.
This significantly reduces the script startup and shutdown times when
there are lots of executables in PATH or lots of entries in the history.
