check: allow use of custom vet when using the language server

[hyangah]

From #50 -
some users had to turn off gopls diagnostics because they need custom vet analyzers.
Find a way for users to turn on the vet.

[gopherbot]

Change https://golang.org/cl/285253 mentions this issue: src/goLanguageServer: partial revert of cl/280601

[tamayika]

I gave up to run go vet with vscode-go and made vscode-any-lint to support multiple lint tools for any files.
Thanks to this extension, I got both gopls's diagnostics and custom analyzer's diagnostics.
So, if there is nobody who wants custom vet with gopls, please close this issue and deprecate custom vet with gopls feature.

[hyangah]

Let's close this - users who want to use a custom vet should consider

• utilize meta linter like golangci-lint and wire it up using the linter setting
• 3rd party extension like @tamayika's vscode-any-lint
• ultimately, gopls should look into a way to make it extensible (see x/tools/gopls: The Need For Gopls Extensibility (Proposal) go#49018)