go: enable completions with placeholders by default (#7780)

This fixes #7523 by enabling completions with placeholders by default.

This setting controls whether gopls sends back snippets with
placeholders. According to the documentation
(https://github.com/golang/tools/blob/master/gopls/doc/settings.md#useplaceholders-bool)
this only controls whether "placeholders for function parameters or
struct fields" are sent in completion responses.

In practice, though, this seems to also control whether any snippets
with *any* placeholders are being sent back.

Example: for the given Go code

    err := myFunction()
    i^

With the cursor being at `^`, this setting controls whether `gopls`
sends back statement snippets such as `if err != nil { return ... }`
with the `...` being dynamically matched to the return value of the
function.

So I think this setting controls far more than just function params and
struct fields. And since we *do* support placeholders in snippets, I
think this provides a better default experience.

Release Notes:

- Improved default Go experience by enabling snippets-with-placeholders
when initializing `gopls`.
([#7523](https://github.com/zed-industries/zed/issues/7523)).