Add Assembly Text Editor

[Matthew-McRaven]

Add the following features & placeholders to the assembler text edit pane:

• (conditional) line numbers
• breakpoints only on "code" lines
• language-dependent syntax-highlighting
• stubs outs for GUI-only error inlays
• Smart tabs for Pep/N assembly language

The code has been moved to a separate library. The library still has some plugin registration issues that will be fixed at a later date.

The following features have been deferred:

• grid select text (select columns and rows; not contiguous from start to end). Usually uses option+arrow keys on MacOS.
• search/replace -- Deferred to a future date.

Closes #387.