Add buffer-close-left, buffer-close-right, and force versions

[john-h-k]

Adds new commands:

bcl, bcl!, bcr, bcr!

which close all buffers to the left or right of the current buffer. Feature in VSCode and some other IDEs and is quite useful.
Not sure what the best way to propose things is so I just made a PR as I already use it on my own version of helix. Happy to close if not wanted

[cor]

Should this also include buffer-close-top and buffer-close-bottom? Might as well add them in one go

[john-h-k]

Should this also include buffer-close-top and buffer-close-bottom? Might as well add them in one go

This closes left/right wrt the buffers listed in the editor, the name order, rather than visual position on screen (in line with how other IDEs have this command)

Would be happy to add those two but:

• Would it be confusing (as those two are spatial, the others aren't)
• Requires more implementation (as it is not just about the document ID list order)