Problem:
Difficult for us to provide default handlers for functions like
`vim.lsp.buf.definition`. When users wanted to fine-tune the default behavior,
they don't know how.
Solution:
- Document an example providing `on_list` boilerplate to make it easier for
users to modify and override.
- Also, considering that the parameters of the previous
`on_list`(`vim.lsp.ListOpts.OnList`) are compatible with the parameters of
`setqflist`, remove that custom type in favor of passing
`vim.fn.setqflist.what`.
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