perf(core): SIMD ASCII fast path for op_decode (#33720)
For pure-ASCII inputs (the dominant real-world case for
`Response.text()`, `File.text()`, FormData parsing, `prompt()` input,
etc.), short-circuit straight to `v8::String::new_from_one_byte` and
skip both the BOM check and V8's internal UTF-8 validation pass.
`v8::simdutf::validate_ascii` is a SIMD high-bit scan (~1 ns per 64
bytes); on non-ASCII inputs it bails at the first non-ASCII byte and
falls through to the existing UTF-8 path.
Same pattern as #33674 (which optimizes `ext/web`'s
`op_encoding_decode_utf8`); this PR optimizes the parallel
implementation in `deno_core`'s `op_decode`, which is the one used by
all the body-decode call sites listed above.
