feat: add MockNow option for deterministic now() in tests

[ChrisJr404]

Fixes #754.

now() makes expressions hard to test because it returns a different value on every run. As suggested in the issue, this adds a small patcher (following the WithTimezone pattern) plus a public option so now() can be pinned to a fixed time.

Usage

program, err := expr.Compile(`now().Sub(StartTime) >= 40*duration("24h")`,
    expr.Env(env),
    expr.MockNow(time.Date(2024, 5, 7, 23, 0, 0, 0, time.UTC)),
)

The compiled program now always sees that fixed time for now(), so tests are deterministic. No boilerplate patcher and no need to DisableBuiltin("now") + re-register a function.

What's included

• patcher.WithNow — replaces now() builtin calls with a constant time node while patching the AST.
• expr.MockNow(t time.Time) — the public option wiring the patcher, mirroring expr.Timezone.
• It composes with Timezone: if a location was injected into now(), the fixed time is returned in that location.
• Docs section under docs/configuration.md.

Notes

• Purely additive; no existing builtins or tests changed, default behavior unchanged.
• Tests cover the basic case, a deterministic derived value (now().Year()), and the Timezone + MockNow combination.
• go build, go vet, and go test ./ ./patcher/ ./builtin/ ./checker/ pass. (test/examples fails to build on a clean master too — unrelated to this change.)