Implement support for n/l modifiers via targets.vim

[adolenc]

targets.vim supports adding custom text objects as plugins, which then support next/last motions. This pull request adds an optional dependency for targets.vim.

Things that I still need to figure out:

• do I really need modFuncs to make the targets be linewise instead of characterwise,
• :checkhealth in neovim now says we have a conflict of mappings (targets.vim and the current mapping),
• count isn't working - v2inT does not select the second next toplevel block.