feat(task): input-based caching with files/output/env (#34509)
Adds input-based caching for `deno task`, addressing #26993. Tasks gain
three new fields: `files` (input globs), `output` (output globs), and
`env` (env var names that are part of the cache key). When a task
declares `files`, Deno fingerprints the command string, appended CLI
args, the values of any listed env vars, and the contents of every
matching file, and skips the task on the next run if nothing changed.
Tasks without `files` keep the existing always-run behavior, so caching
is opt-in per task.
On a cache hit, declared `output` globs are restored from the cache, so
deleting an output (e.g. `dist/`) and re-running regenerates it; stale
outputs from a prior run are cleared before a re-run. Each task's direct
dependency fingerprints are folded into its cache key, so a downstream
task re-runs when an upstream one did, and a non-cacheable dependency
propagates non-cacheability downstream.
The fingerprint uses SHA-256 (length-prefixed fields) so a collision
can't silently skip a build, and folds in the target OS/arch and Deno
version so a DENO_DIR synced across platforms or upgrades never hits
across them. A mtime+size fast path lets a hit skip reading input files
entirely unless a stat drifts, falling back to rehashing contents (which
distinguishes a real edit from a mtime-only touch). Cache entries live
under `DENO_DIR/task_cache_v1/` as a small JSON manifest; an
unrecognized payload is treated as a miss, so no migration is needed.
Refs #26993
