Templater doesn't have the one yet, but I think it belongs to the same
category.
For clap::Error, we could use clap's own mechanism to render suggestions as
"tip: ...", but I feel "Hint: ..." looks better because our error/hint message
is capitalized.
This commit moves the parse_string_pattern helper function into the
str_util module in jj lib and adds tests for it.
I'd like to reuse this code in a function defined by `UserSettings`, which is
part of the jj lib crate and cannot use functions from the cli crate.
