[DRAFT] feature: sync mark region for vi-mode.

[sakurawald]

Related issue: #1590

To sync the mark-region between vi-mode and mouse-clicking:

1. Ensure the vi-visual-char mode is entered, when selecting a region using mouse.
2. Sync the *start-point* for vi-visual-mode.

This pr contains some cons:

1. The visual-overlay may not be cleared correctly: The visual-overlay will be cleared after you pressed y key in vi-visaul mode, if (current-point) > *start-point* (In right-side). It will not be cleared if (current-point) < *start-point* (In left-side).
2. Press v key again in vi-visual-char mode will not clear the visual-overlay selected by mouse.

Anyway, this pr:

1. Ensure the vi-mode will be vi-visual-char mode, if you use mouse to select a mark region.
2. Press y key in vi-visual-char mode can copy the correct string into clip-board. (Though it will not ensure the visual-overlay be synced.)

[sakurawald]

This impl only work in lem self-hack mode, and failed to pass the compilation.

The main point is to:

1. If a mark region is selected by mouse-clicking, then enter vi-visual-char mode automatically.
2. Set the *start-point* for vi-visual-char mode.
3. Ensure the visual-overlay in vi-mode will be cleared after be consumed, e.g. after v key pressed in visual mode or after y key pressed in visual mode.

There are some tasks should be done:

1. Extract the vi-mode specific code from the core code, to avoid the access of lem-vi-mode package.
2. Maybe use a hook to listen the mark region changed event, and set the *start-point* for vi-visual-char mode.
3. The visual-overlay in vi-mode didn't disapper in some cases, like if the (current-point) < *start-point*, then the visual-overlay will not be cleared, and requries extra esc key to clear this overlay.