refactor!: completion should only be trigger by `[[`.

[neo451]

I think with #474 started, we can make completion only be triggered by [[ for both markdown and wiki links, like obsidian app, it will be less intrusive for typing

case: maybe you want to type something like table[field] as a simple codespan in your notes learning lua, just using [ symbol would trigger the completion, while [[ is really clearly just a link, not often used in other contexts.

Also currently [ triggers markdown, and [[ triggers wiki, but it don't respect the preferred link style.