Add limited support for embedded APL sequences
Add limited support for embedded APL sequences

By "limited", I mean the start/end delimiters of embedded APL blocks are
*not* set dynamically based on a dyadic ⎕INP's left-hand arguments list.
While I thought it'd be possible with some clever nesting or lookaheads,
there's no way to interpolate nested patterns using delimiters set by an
earlier match's capturing groups.

So I'm settling for a more limited solution: using hard-coded tag types.
These are currently limited to the following (case insensitive):

    <?apl ?>   <?= ?>   <? ?>   # PHP-style
    <%apl %>   <%= %>   <% %>   # ASP.net-style

These are arguably the least likely to trigger inadvertent highlighting,
and are the most logical choice, given their semblance to PHP.

Alhadis/language-apl