add 2 snippets: impstd & importstd and import std module url completion

[CGQAQ]

$0: denotes the final cursor position
$1..n: tabstops in sequence

impstd => import {$3} from 'https://deno/std@$1/$2';$0"
importstd => import (foo|{foo}| * as foo) from 'https://deno/std@$1/$2';$0"

• fetch std library information dynamically from github
• import completion
• std version completion
• cache the result
• add command to clear the cache to allow user to refresh local import std cache

Screenshots:

Implementation of #114

[lucacasonato]

@CGQAQ Thanks, this is a cool feature. Since you implemented this a lot in this repository has changed, and also there has been a complete rewrite of the deno.land/x registry powering deno.land/std, so this PR will need some changes. If you want to continue that is cool, otherwise I can pick up this PR and finish it.

I think this feature could be extended to support all of deno.land/x, not just std with these new API endpoints on the registry:

• https://cdn.deno.land/$MODULE/meta/versions.json to get a list of versions for a module
• https://cdn.deno.land/$MODULE/versions/$VERSION/meta/meta.json to get a file tree of the module
• https://api.deno.land/modules?limit=10&query=$QUERY to search deno.land/x

Also I think we should limit our cache to a max of 1 hour, after which we would lazily fetch new versions automatically.

[CGQAQ]

@CGQAQ Thanks, this is a cool feature. Since you implemented this a lot in this repository has changed, and also there has been a complete rewrite of the deno.land/x registry powering deno.land/std, so this PR will need some changes. If you want to continue that is cool, otherwise I can pick up this PR and finish it.

I think this feature could be extended to support all of deno.land/x, not just std with these new API endpoints on the registry:

• https://cdn.deno.land/$MODULE/meta/versions.json to get a list of versions for a module
• https://cdn.deno.land/$MODULE/versions/$VERSION/meta/meta.json to get a file tree of the module
• https://api.deno.land/modules?limit=10&query=$QUERY to search deno.land/x

Also I think we should limit our cache to a max of 1 hour, after which we would lazily fetch new versions automatically.

OK, I'll change it, thank you for your information