x/tools/gopls: support language features for text/template, html/template

[stamblerre]

See discussion on microsoft/vscode-go#228.

[gopherbot]

Change https://golang.org/cl/301493 mentions this issue: text/template/parse: add mode to skip func-check on parsing

[gopherbot]

Change https://golang.org/cl/301493 mentions this issue: text/template/parse: add a mode to skip func-check on parsing