x/tools/gopls: feature request: support more postfix snippets for primitive type

[xieyuschen]

Feature I want

Gopls has provided some postfix snippet, for example:

I believe we can add more useful postfix snippets:

1. []byte!: help to convert a string to a slice of bytes.
2. string!: help to convert a slices of bytes to a string.
3. T!: help to convert a type from another, the postfix will generate |(variable) for users to input a type they want. | is the cursor position. Further more, we may provide some primitive type if the variable is a primitive(float, int, uint, etc...).
4. itoa!: help to convert an int to string
5. more cases...

Indeed we can provide a lot of helpful postfix snippets, not only the cases I mentioned above. Please kindly let me know your opinions about it. @adonovan @findleyr
I want to contribute these small features.

[gopherbot]

Change https://go.dev/cl/666195 mentions this issue: gopls/internal/golang/completion: tostring, tobytes postfix snippets

[findleyr]

We should discuss this in our triage meeting today. The additions seem reasonable, but I want to avoid an combinatorial explosion of little helpers. I'm not sure we have a theory for what belongs as a snippet.