This refactors the behavior of string match with capture groups to
correctly handle multiple arguments. Now the variable capture applies to
the first match, as documented. Fixes#7938.
string match is documented as setting an unset variable if a capture group
is unmatched in an otherwise matched regex, and if the `--all` flag is not
provided. However prior to this fix, it instead set a variable containing
the empty string as a single value. Correct the implementation to match
the documentation.
Note that if the `--all` flag is provided we continue to set empty
strings, which is documented.
The '--import' flag was used for importing named capture groups, but it
was decided to always import them unconditionally. This flag was causing
the tests to fail.
