Use local files on connection failure, closes #298

[WorldSEnder]

Use already downloaded tools when fetching latest release metadata fails. This adds a small cache file in the globalStoragePath containing the last fetched information.

Does not currently implement the feature as requested in #281 as I'm not certain what exactly such a flag should do (pick one):

• Not download anything at all
• "haskell.hlsUpdateBehavior": "keep-up-to-date" Try to fetch latest release information, if that fails fall back to local cache (what is done with this PR automatically)
• "haskell.hlsUpdateBehavior": "prompt" Try to fetch latest release information, but not upgrade, even if it succeeds and instead display a prompt to update (probably sensible), still download artifacts for the proper ghc version, if necessary.
• "haskell.hlsUpdateBehavior": "never-check" Don't check for latest release information, and do not upgrade, but still download artifacts for missing ghc versions, if needed.

I would want to implement #281 in this PR if that behavior is cleared up for me :)

[WorldSEnder]

Ping @Meowcolm024 @runeksvendsen I would love to hear your feedback on this if you have time

[jneira]

Very much thanks for the pr, it will improve the user experience a lot.
The last one (display a prompt) seems to be more complete and user friendly, similar to how vscode itself does the updates.
I would not ask if no version is available cause it would make the extension fail to load any haskell content.
However second and third are complementary, right? Prompt if it is succesful and fall back to previous version automatically if it is not (with a specific waning).

[WorldSEnder]

The difference between the second and the third option is that the former is implemented, the latter would require some extra work - which I'm willing to put in, in any case.

[WorldSEnder]

Another point: I'm reading #272 and it seems the current code could use some validation of the json result returned from github, since it, as written, now writes a potentially malformed result to disk. I first thought to maybe use a simple json schema to validate, but after researching the vscode environment, it seems one would have to include rather big dependencies to do that.

Something like avj is quite a big dependency, a little bit smaller io-ts, which at least doesn't have any transitive dependencies. Then there's two very small and also small but probably not well maintained libraries. Honestly, I'd rather just write the logic directly inline. Any opinions on that?

[jneira]

I would say that inline would be better if the code is small enough, but I am not an expert in js/ts

[WorldSEnder]

Alright, this should now also address #272 (by doing validation on the data received from github) and #281 (by adding a configuration item that can be set to prompt before deciding to use a newer version)

[lukel97]

Sounds like 5158096 also fixes #281, I'm happy with this design. This all LGTM, will quickly test it out locally

[lukel97]

Thanks for doing this, looks great