x/tools/gopls: add a method snippet to postfix completion

**Is your feature request related to a problem? Please describe.**
Typing out methods is tedious because one always has to fill the receiver field. The `meth` snippet is great but this would be even better.

**Describe the solution you'd like**
It would be real slick to have a completion from:

```go
type X struct {}

X.meth!
```
to
```go
type X struct {}

func (x X) _() {
}
```

