gopls/internal/telemetry/cmd/stacks: refactor report processing to support different programs

Only gopls is actually supported for now. This CL is intended to be a
no-op. It consists of two main pieces:

* Refactoring the giant main function into smaller pieces that are
  easier to understand individually.
* Adding a ProgramConfig to describe how to process reports from a
  specific program.

Updates golang/go#71045.

Change-Id: I6a6a636c5b8b56bf72354a8795320eac7de7ef93
Reviewed-on: https://go-review.googlesource.com/c/tools/+/642419
Reviewed-by: Alan Donovan <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: prattmic