Missing support of `,.` in backquote

[cireu]

In https://www.lispworks.com/documentation/HyperSpec/Body/02_df.htm

Anywhere ,@'' may be used, the syntax ,.'' may be used instead to indicate that it is permissible to operate destructively on the list structure produced by the form following the ``,.'' (in effect, to use nconc instead of append).

An example in SBCL.

CL-USER> `(a b c ,.(list 1 2 3))
(A B C 1 2 3)

In theory, it's ok to re-use the code of ,@ to implement ,. -- standard just allow implementation to use destructive splicing, not require.

[kchanqvq]

Let's not talk about compliance of read.lisp for now 🤫