Add menus to M2-mode and M2-comint-mode

[d-torrance]

These menus may be useful for users to discover what functions are available in the two Macaulay2 major modes and to quickly learn their keybindings.

The menus include all functions marked interactive. Functions that have a corresponding keybinding in M2-mode only are only displayed in the M2-mode menu, and similar for M2-comint-mode. Functions that have a corresponding keybinding in both modes are displayed in both menus.

There are some exceptions to this:

• M2 is bound to f12 in both modes, but really only makes sense in the M2-mode menu.
• M2-send-to-program is bound to f11 in both modes, but makes the most sense in the M2-mode menu, as M2-send-to-program-or-jump-to-source-code serves a similar purpose in M2-comint-mode.
• M2-dynamic-complete-symbol is marked obsolete, so it is omitted.
• M2-electric-right-brace has its keybinding commented out, but we include it for completeness.
• M2-electric-tab is set to indent-line-function for both modes, but TAB is bound to completion-at-point in M2-comint-mode, so we only include it in the M2-mode menu.

Screenshots

M2-mode

M2-comint-mode