FillStruct using gopls (feature request)

[ubombi]

According to golang go 37576 , golang tools 220 and googlesource 227437

:GoFillStruct is now implemented in gopls. So we can use it, instead of davidrjenni/reftools/cmd/fillstruct

[bhcleek]

I'll keep an eye on the gopls releases and will likely update vim-go to use gopls for fill struct after a release of gopls that includes cl 227437.

[ubombi]

Released in v0.4.2