Fix double expansions (`$$foo`)

Double expansions of variables had the following issues:

* `"$$foo"` threw an error no matter what the value of `$foo` was.
* `set -l foo ''; echo $$foo` threw an error because of the expansion of
  `$foo` to `''`.

With this change, double expansion always works properly. When
double-expanding a multi-valued variable, in a double-quoted string the
first word of the inner expansion is used for the outer expansion, and
outside of a quoted string every word is used for the double-expansion
in each of the arguments.

    > set -l foo bar baz
    > set -l bar one two
    > set -l baz three four
    > echo "$$foo"
    one two baz
    > echo $$foo
    one two three four

1 files changed, 1 insertions, 0 deletions

diff --git a/tests/expansion.status b/tests/expansion.status
new file mode 100644
index 00000000..573541ac
--- /dev/null
+++ b/tests/expansion.status
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+0