go/analysis/passes/printf: extract operation parsing logic into tools/internal/fmtstr

This CL made a refactor that extract operation parsing logic into
tools/internal/fmtstr package, so they can be used uniformly by
printf-analyzer for type checking, DocumentHighlight, and future
possibility for SemanticHighlight and rich Hover information with little effort.

Previously, the code responsible for parsing and validating printf-style
format strings was spread inline and blend together in printf.go,
in order to use it for documenthighligt/hover, this CL extends formatState
by encoding posRange and metainfo for every sub-item of a operation
(flag, width, precision, verb), and only expose those field, by that way
callers can compute necessary information from them for various usages.

Updates golang/go#70050

Change-Id: I8337ad0bdf9f5c1aa301d2e9155b2d4938cf4a7c
GitHub-Last-Rev: 8e57634
GitHub-Pull-Request: #547
Reviewed-on: https://go-review.googlesource.com/c/tools/+/632598
LUCI-TryBot-Result: Go LUCI <[email protected]>
Commit-Queue: Alan Donovan <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>
Auto-Submit: Alan Donovan <[email protected]>

Author: xzbdmw