Bug 27747: Add CodeMirror custom syntax highlighting for column placeholders

Bug 27747: Add CodeMirror custom syntax highlighting for column placeholders

This patch adds some additional configuration to CodeMirror so that
column placeholders have a distinct color in the CodeMirror SQL editor.

To test, apply the patch and create or edit an SQL report which contains
one or more column placeholders, e.g. [[itemnumber|Item number]],
[[borrowernumber|Borrower number]], etc.

Confirm that when editing the SQL, these placeholders should appear as
red text. Save your report and view it. The syntax highlighting should
be updated in this view too.

Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Lucas Gass <lucas@bywatersolutions.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>