Replace purescript-decl-scan-mode from purescript-mode

[bjarkevad]

While enabling purescript-decl-scan-mode makes imenu queries possible, the solution isn't very robust as it depends on a bunch of regexps. A better solution would be querying psc-ide for a list of declarations in the current buffer.

cc: @kritzcreek

Related discussion:
syl20bnr/spacemacs#9648

[kritzcreek]

I'm not entirely sure which functions we need to override/implement to hook into imenu, but looking at the description of the feature in the spacemacs issue we should be able to get all the information we need from these two endpoints:

• Imports: https://github.com/purescript/purescript/blob/master/psc-ide/PROTOCOL.md#imports
• Declarations: https://github.com/purescript/purescript/blob/master/psc-ide/PROTOCOL.md#complete together with a filter for the current module. We might even want to think about using the declaration filter, to separate types from values in the list.

Edit: Actually... at least for the import declarations we don't get sourcespans from the endpoint. I'm not sure how important these are, and in case they are it shouldn't be too hard to find them with a simple regex.