feat(diagnostic): fromqflist({merge_lines}) #37416

Problem:
`vim.diagnostic.fromqflist` ignores lines that are `item.valid == 0` (see
`getqflist`). Many qflists have messages that span multiple lines, which look
like this:

    collection/src/Modelling/CdOd/Central.hs|496 col 80| error: [GHC-83865]
    ||     • Couldn't match expected type: InstanceWithForm
    ||                                       (FilePath
    ||                                        -> SelectValidCdInstWithForm
    ...

calling `vim.diagnostic.fromqflist(vim.fn.getqflist)` gets a diagnostic message
like this:

    error: [GHC-83865]

only the first line is kept, but often, the remaing lines are useful as well.

Solution:
Introduce `merge_lines` option, which "squashes" lines from invalid qflist items
into the error message of the previous valid item, so that we get this
diagnostic message instead:

    error: [GHC-83865]
         • Couldn't match expected type: InstanceWithForm
                                           (FilePath
                                            -> SelectValidCdInstWithForm

0 files changed, 0 insertions, 0 deletions