feat: precede opened or related buffers

[delphinus]

Fix #272

This add a feature to precede relational entries to already opened ones.

preceding = nil (default value)

This is the original behavior. It does not precede any entries and just sort by score.

100  /path/to/project_A/index.html
 80  /path/to/project_B/Makefile
 60  /path/to/project_A/image.jpg
 40  /path/to/project_C/lib/main.ts
 20  /path/to/project_B/main.c
 10  /path/to/project_A/style.css

preceding = "opened"

When you opened project_A/image.jpg and project_B/Makefile, it precedes them against other candidates.

 80  /path/to/project_B/Makefile
 60  /path/to/project_A/image.jpg
100  /path/to/project_A/index.html
 40  /path/to/project_C/lib/main.ts
 20  /path/to/project_B/main.c
 10  /path/to/project_A/style.css

This is like :Telescope buffers because filenames in existent buffers are above others.

preceding = "same_repo"

When you opened the same files above, it precedes candidates that are under the same repositories.

100  /path/to/project_A/index.html
 80  /path/to/project_B/Makefile
 60  /path/to/project_A/image.jpg
 20  /path/to/project_B/main.c
 10  /path/to/project_A/style.css
 40  /path/to/project_C/lib/main.ts