use interactive text editor in frontend

**Is your feature request related to a problem? Please describe.**
When debugging a program, the program cannot be modified in the gdbgui interface. It has to be opened else where, then saved. This can be cumbersome.

**Describe the solution you'd like**
Use an interactive code editor in the web UI, such as Microsoft's [Monaco](https://microsoft.github.io/monaco-editor/) as a text editor instead of hand-made React component. This essentially enables all of VSCode's language service capabilities in the browser, such jump to definition, language-specific semantic understanding of variables for things like hover. Adds ability to edit+save files. Also must support inline disassembly display.

**Describe alternatives you've considered**
Use a text editor or IDE.

No files or tests are named. Start by locating the hand-made React editor component and the frontend entry points that display source and disassembly; the requested result is an interactive browser editor supporting editing, saving, language-service features, and inline disassembly.

Written by the indexing model from the issue text.