fix(statusline): display null-ls linters properly

[choplin]

Description

null-ls linters may have either DIAGNOSTIC, DIAGNOSTIC_ON_OPEN, or DIAGNOSTIC_ON_SAVE as its method parameter. e.g., https://github.com/jose-elias-alvarez/null-ls.nvim/blob/8914051a3d399e9715833ad76bbf5fe69ea7faf0/lua/null-ls/builtins/diagnostics/buf.lua#L12

Currently, only linters with DIAGNOSTIC are shown in the lsp component on the statusline regardless of whether the linters are registered and work properly.

This is due to lvim.lsp.null-ls.linters.list_registers returning only linters with DIAGNOSTIC.

This PR makes that function retrieve the linters with all three method types and return the merged list.

How Has This Been Tested?

• Register buf null-ls linter, which is set to DIAGNOSTIC_ON_SAVE, open a supported file (i.e., proto file), then check if buf linter is shown in the statusline.

[choplin]

null-ls formatters may have an alternative method as RANGE_FORMATTING as well. But, it seems that no built-in formatters of null-ls have only RANGE_FORMATTING method (all formatters that have RANGE_FORMATTING also have FORMATTING method). So, I didn't make changes to formatters.