go/analysis/passes/modernize: add slicesbackward analyzer

Adds a new slicesbackward pass to the modernize suite that suggests
replacing manually-written backward loops over slices with
slices.Backward (Go 1.23).

Before:

    for i := len(s) - 1; i >= 0; i-- {
        use(s[i])
    }

After:

    for _, v := range slices.Backward(s) {
        use(v)
    }

The analyzer matches for loops with the exact pattern:
- init:  i := len(s) - 1  (where s is a slice expression)
- cond:  i >= 0
- post:  i--

When every use of i in the body is s[i], those are replaced with a
fresh value variable and the range clause becomes "_, v :=".

When i is used for other purposes too, both index and value are kept:
"i, v := range slices.Backward(s)".

Loops where i is assigned or address-taken inside the body are skipped
to avoid changing program behavior. Loops using = (not :=) in the init
where i is address-taken before the loop are also skipped.

The analyzer is unexported; it is accessible via goplsexport until the
proposal is approved and the API is published.

Fixes golang/go#78484

Change-Id: Ic920cc149c4a3ff2524f4d17f347ec48ab43d05a
GitHub-Last-Rev: 4b3470ab6b3edb7c7492742b1255bb2f0777b2be
GitHub-Pull-Request: golang/tools#627
Reviewed-on: https://go-review.googlesource.com/c/tools/+/761740
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
Reviewed-by: Junyang Shao <shaojunyang@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>