Implement DEP 12, string literals supporting Rectangle Rule

See https://opendylan.org/proposals/dep-0012-string-literals.html#the-rectangle-rule
for details.

documentation/library-reference/source/language-extensions/string-literals.rst

@@ -9,11 +9,15 @@ processing ("raw" strings) and multi-line strings. Briefly,
 #. Multi-line strings begin with three double-quote characters: ``"""``
 
 #. End-of-line sequences in multi-line strings are always parsed as a single
-   Newline (``\\n``) character, regardless of source file line endings or the
+   Newline (``\n``) character, regardless of source file line endings or the
    conventions of the operating system.
 
 #. Any string, whether delimited by ``"`` or ``"""`` may be prefixed with
    ``#r`` or ``#R`` to disable escape sequence processing.
 
+#. Any leading whitespace that *matches the whitespace preceding the end
+   ``"""`` delimiter* is removed, allowing multi-line strings to be formatted
+   nicely in source code.
+
 See `DEP 12 <https://opendylan.org/proposals/dep-0012-string-literals.html>`_
 for details.