Improve CSS and GeSHi blocks to support loading `src` from a file

[bobbingwide]

Using the shared library logic developed in https://github.com/bobbingwide/oik-bob-bing-wide/issues/40 for the CSV block, apply similar changes to the [bw_csv] and [bw_geshi] shortcodes and their associated blocks oik-css/css and oik-css/geshi.

Requirements

• Improve the support for loading content from a source file rather than being entered into the block
• Improve the blocks to add a text field used to identify the file to be loaded.

Proposed solution

• [x] Implement the shared library file libs/class-oik-attachment-contents.php
• [x] Add a text field in the Settings area to accept the attachment ID, URL or file name
• [ ] Update documentation with appropriate examples

[bobbingwide]

Note: The PHPUnit test case for bw_geshi__example() has been wrong for quite a while. This will need to be corrected.

[bobbingwide]

oik-css v1.3.0 was released to oik-plugins on 16th May. This minor version was not available on wordpress.org oik-css v2.0.0 was packaged on 31 Aug and is now on wordpress.org There's no documentation for the src attribute... But I have used the attribute in the wild.