Fix completion of markdown links with LSP titles

[vandalt]

Fixes #435 and zk-org/zk-nvim#176.

After looking at the code and testing earlier versions, it seems that the issue was introduced in #397, which fixed the issue of triggering completion manually, but also introduced offsets that account for words instead of just using a -2 offset. The -2 offset worked for both [[ and ((, but the checks in #397 only accounted for [[ which broke completion for ((.

This PR just adds an extra condition to also catch ((. Some notes:

• I don't really know go, so comments welcome
• It seems that trigger (non-auto) completion with [[| was fixed by fix(lsp): fix trigger completion of zk LSP #397, but that [[somewords| still does not work. And it still does not work with these small updates (but ((| works just like [[)
• I'm marking this as a draft for now because I want to double check the TODO comments I added (basically (( is included in the word regex, but [[ is not, so I need to account for that

[vandalt]

Ok I think this is ready for review. The second bullet point above is also fixed: invoked (non-auto) completion now also works with parentheses.

Thanks!

[tjex]

Thanks for all of this, and sorry for being slow. I'm juggling a lot atm, but will cool off round July / August

[tjex]

Tested with and without autopairs enabled. Tested manual completion as well.

All working and code changes seem straight forward enough, thanks!