feat: render bufferline separator

[ayn2op]

Preview:

Configuration:

[editor.bufferline]
render-mode = "multiple"
separator = "+"

Closes #11372

[poliorcetics]

You should probably add a screenshot or two to the MR desc so we can have an idea of what happens :)

[ayn2op]

You should probably add a screenshot or two to the MR desc so we can have an idea of what happens :)

With default separator and multiple as render mode: