Move "cursor snap" logic from cursor_jump() into a utility function

[Kiyoshika]

I feel like I've implented this logic three different ways throughout this library, and I think the cursor_jump() implementation is probably the cleanest. Just move this into a utility maybe called cursor_snap() which will move the cursor to a specific row/column position and adjust scrolling if needed.

For reference, below is a paste of the implementation from cursor_jump()

NOTE: this was pasted directly from vim so I can't really format it...

NOTE 2: row is the new proposed row (could be less than or greater than cursor->row, or the same) and column is the proposed column.

// TODO: move the cursor scrolling logic to a utility function.
3             // this can be used in other places since I feel like I've implemented
2             // this in three different ways by now...
1             if (screen->lines[row][column] == search_char)
352             {
1                 // NOTE: the +2 is to account for "lines" being 1-indexed and
2                 // the bottom line is inaccessible (used to display edit mode)
3                 if (row > screen->end_idx)
4                 {
5                     screen->end_idx = row + 2;
6                     screen->start_idx = screen->end_idx - screen->max_rows;
7                     screen->current_line = screen->max_rows - 2;
8                 }
9                 else if (row < screen->start_idx)
10                 {
11                     screen->start_idx = row;
12                     screen->end_idx = screen->start_idx + screen->max_rows;
13                     screen->current_line = 0;
14                 }
15                 else
16                 {
17                     if (cursor->row > row)
18                         screen->current_line -= (cursor->row - row);
19                     else
20                         screen->current_line += (row - cursor->row);
21                 }
22
23                 cursor->row = row;
24                 cursor->column = column;
25
26                 move(screen->current_line, cursor->column + cursor->line_num_size + 1);
27                 screen_draw(screen, cursor);
28                 return;
29             }