Supporting different ghost_text highlight when selected

[Suryansh-Dey]

Problem

I had a visual problem in ghost text when using rust-tools, since it does pre-selects sometimes and sometimes not when no preferred completing item is available.
Now with current ghost text view, I can't know if it's the item is from the first item in the table or a pre-selected one:
Consider following images:

not pre-selected

pre-selected

But the key difference is, in first instance, I need to select (Tab) then confirm (hit Enter) to get the ghost to real text while in second instance, I need hit enter stright up! Hitting Tab + Enter give wrong item!

To handle this, I need to keep checking the large completing menu to know if the ghost text item is pre-selected or not.
Hence ghost-text fails to achieve the actual moto i.e. accept completing without moving your eye off from the cursor.

Solution

I have made a slight change to allow to use different highlight group for not pre-selected ghost text (i.e. text due to first item in the menu) and pre-selected ghost text as you can see below

not pre-selected

pre-selected

Config used:

M.experimental = {
    ghost_text = {
        hl_group = 'Comment',
        hl_group_selected = "CmpItemAbbrMatch"
    }
}

Change

• I just introduced one more option in M.experimental.ghost_text i.e. hl_group_selected (can be nil for backward compatibility) that applies on ghost-text when item is selected in completion menu.
• If hl_group_selected is not used or nil, everything works as it used to do before.