feat: add Skip middleware; improve docs

[lrstanley]

Changes in this PR

• Added httprate.Skip, which is a simpler (and more obvious) shorthand for setting the increment to 0 in context, to have endpoints designed to let API users see what their rate limit is (specifically when the key isn't endpoint specific, for example), without incrementing it. i.e., an endpoint that still sets the associated headers, without incrementing the rate limit.
  • Added test that covers this new flow.
• Added documentation/comments to most functions, as there were almost none before.
  • Uses the newer linkable syntax for inter-package and external-package types (in most IDEs, allows hovering over the function/type, and making those type references clickable).
• Adjusted some of the logic to be modernized for Go 1.24 -- loop vars, range over int, time.Time.Equals for more accurate time equality checks, etc.
• Fixed odd formatting of the go.mod file.
• More stdlib constant usage vs generic ints, e.g. 429 vs http.StatusTooManyRequests.

Notes to Reviewer

All tests pass locally with:

go test -v -race -count 10 -cpu 1,4 ./...

[github-actions]

Benchmark Results

goos: linux
goarch: amd64
pkg: github.com/go-chi/httprate
cpu: AMD EPYC 7763 64-Core Processor                
               │ master.txt  │            pr.txt             │
               │   sec/op    │   sec/op     vs base          │
LocalCounter-4   32.57m ± 4%   32.84m ± 1%  ~ (p=0.529 n=10)

               │  master.txt  │             pr.txt             │
               │     B/op     │     B/op      vs base          │
LocalCounter-4   2.845Mi ± 0%   2.847Mi ± 0%  ~ (p=0.280 n=10)

               │ master.txt  │            pr.txt             │
               │  allocs/op  │  allocs/op   vs base          │
LocalCounter-4   121.6k ± 0%   121.6k ± 0%  ~ (p=0.289 n=10)