HLS prioritization and product development

Since we have a contract with WT and a number of contributors, we should start being a little more product oriented and create a priority list.

E.g. my opinion on the top 3 features missing are:

1. https://github.com/haskell/haskell-language-server/issues/708
2. https://github.com/haskell/haskell-language-server/issues/709
3. a plugin for adding new modules to a cabal file (briefly discussed with @fendor ... seems possible and would make e.g. hpack obsolete, as well as many odd requests to cabal)

For non-functional priorities, I'd say:

1. performance of HLS (including memory footprint)
2. better/swifter releases (lock-step with GHC)
3. cross-compiler support (new ghcjs... was this discussed? Does it even work?)

So far, I propose to discuss priorities and write them down in the readme or contribution document. Then figure out what the status of each of those is, what blocks them and if we have enough volunteers or funds to execute some of it.

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@wz1000 @michaelpj @fendor @pepeiborra @david-christiansen @Kleidukos