Pass file to format to custom formatter

[mrmeku]

Describe the bug

Currently the vscode-go extension has code paths to support running a custom go formatting tool. However, those code paths stop short of actually passing the file that should be formatted as an argument to the formatter. This makes it so that a custom formatting tool cannot actually be run since it has no way of knowing what file it should format

Steps to reproduce the behavior:

1. Set "go.formatTool" to a path to a custom tool
2. Attempt to format a go file
3. See that that the custom tool is not passed an argument of a file to format

[gopherbot]

Change https://golang.org/cl/332529 mentions this issue: src/goFormat.ts: pass filename to custom formatters

[hyangah]

It's tricky to assume what a "custom" formatter takes as args. Is it not possible to utilize "go.formatFlags"? Maybe implement variables replacement similar to vscode's launch.json/task.json variables if necessary?

The extension runs the format tool inside the directory of the document (assuming go fmt style behavior that takes the package path or the package in the current directory)

[mrmeku]

My intention is that you would wrap the custom formatter in a script so that you can transform why the extension passes into the form the formatter needs.

Also happy to introduce a template variable if that's preferred

[hyangah]

@mrmeku We discussed this in our internal team meeting and we hope to stick with the go fmt style that takes the optional 'packages' argument, in order to avoid breaking other existing use cases. Can you please change to support variable substitutions for go.formatFlags? Thanks!

[gopherbot]

Change https://go.dev/cl/446298 mentions this issue: docs/settings: make the custom formatter support more visible

[gopherbot]

Change https://go.dev/cl/446296 mentions this issue: test/gopls: narrow the scope of fake formatTool setting