Add support for PL/SQL

### Is your feature request related to a problem? Please describe
A user just wants to be able to add properly highlighted PL/SQL

### Describe the solution you'd like
Add proper syntax highlighting for PL/SQL

### Describe alternatives you've considered
I told them they could try choosing SQL for now. They were just using plain text. I tested with a few samples from [this site](https://docs.oracle.com/cd/A97630_01/appdev.920/a96624/a_samps.htm) with SQL selected, and some of it is picked up nicely, but other parts (print and what not for example) are not highlighting as one might expect:

### Additional context
Here's the original request:
https://wordpress.com/forums/topic/plsql-code-is-not-available-in-syntaxhighlighter-block/

The user had just selected text for now.

The issue names no files, tests, or entry points. Start by locating the existing SQL highlighting implementation and compare it with the PL/SQL samples linked in the issue. Done means PL/SQL is properly highlighted instead of being treated as plain text.

Written by the indexing model from the issue text.