CBM basic: allow export the current BASIC memory to text file and to clipboard

[lgblgblgb]

Develop a shared component which allows CBM-like BASICs to export current BASIC memory content to a selected file, or to the clipboard. Optionally, allow to do it, into a Tex format, easy to embed then into things like the Mega65 documentation project. However, this should be a shared component probably usable with various machines (most notable: C65, but also maybe VIC20) in Xemu, using CBM BASIC.

[lgblgblgb]

Commit 24e4fa35807181e9a4b2c22939c3f6ddae6ec110 has some infrastructure.