Fix link autocompletion with tags

[memeplex]

Fix #884

Alternative PR: #886

[memeplex]

@riccardoferretti what do you think of simplifying the regular expressions a bit, like this:

export const WIKILINK_REGEX = /\[\[[^[\]]*$/;
export const SECTION_REGEX = /\[\[([^[\]]*#)[^[\]]*$/;

Instead of the negative lookahead I'm requesting a prefix that starts with [[ and extends to the cursor position without any [ or ] in between.

Moreover, an alternative fix could be to exclude the # case in the regex itself:

export const WIKILINK_REGEX = /\[\[[^[\]#]*$/;

I think I prefer this one.

[riccardoferretti]

Nice and clean. Can we add a test case for it?

[memeplex]

Can we add a test case for it?

I've added some ones.