Allow overriding 'parseword' to make '...' acceptable syntax

[masak]

In the "main language", writing ... doesn't work, because the reader doesn't know what to do with it:

Language::Bel 0.51 -- msys.
> '(new-inst %class-exp %rand ...)
Error: final-intrasymbol at lib/Language/Bel/Reader.pm line 316, <> line 1.
>

In a world with a bootstrapped reader, this could be fixed like this:

diff --git a/pg/bel.bel b/pg/bel.bel
index 5479289..82d06c4 100644
--- a/pg/bel.bel
+++ b/pg/bel.bel
@@ -1217,6 +1217,7 @@
 (def parseword (cs base)
   (or (parsenum cs base)
       (if (= cs ".")       (err 'unexpected-dot)
+          (= cs "...")     'ellipsis
           (mem \| cs)      (parset cs base)
           (some intrac cs) (parseslist (runs intrac cs) base)
                            (parsecom cs base))))

• (a) add a test that uses the userspace reader, to make sure it fails but then succeeds after being extended
• (b) add a to-do test that checks the same with the built-in reader