perf: minimize identifier lookahead when parsing let

[JLHwung]

Q	A
License	MIT

When reviewing #13325 I realize that we always peek a complete identifier after let in order to compare with in and instanceof. However, we could exit early if the next character is an IdentfierStart and from this position on it does not match in or instanceof.

In a moderate let + length-10 identifier (not starting with i) benchmark cases, I see 17% performance gain. This benchmark is included in this PR.

node --predictable ./10-length-binding.mjs
baseline 64 let and length-10 binding identifiers: 5257 ops/sec ±45.3% (0.19ms)
baseline 128 let and length-10 binding identifiers: 3482 ops/sec ±0.28% (0.287ms)
baseline 256 let and length-10 binding identifiers: 1712 ops/sec ±0.75% (0.584ms)
baseline 512 let and length-10 binding identifiers: 848 ops/sec ±0.34% (1.179ms)
current 64 let and length-10 binding identifiers: 6109 ops/sec ±49.27% (0.164ms)
current 128 let and length-10 binding identifiers: 4018 ops/sec ±0.93% (0.249ms)
current 256 let and length-10 binding identifiers: 2005 ops/sec ±0.36% (0.499ms)
current 512 let and length-10 binding identifiers: 966 ops/sec ±0.89% (1.036ms)

If we go to extreme and benchmark with length-100 identifier, the performance gain is 30%. This is expected since it completely avoid lookahead 100 characters to compare with in or instanceof. The benchmark is not included in this PR.

baseline 64 let and length-100 binding identifiers: 2934 ops/sec ±45.04% (0.341ms)
baseline 128 let and length-100 binding identifiers: 2036 ops/sec ±1.22% (0.491ms)
baseline 256 let and length-100 binding identifiers: 1003 ops/sec ±1.21% (0.997ms)
baseline 512 let and length-100 binding identifiers: 484 ops/sec ±0.95% (2.067ms)
current 64 let and length-100 binding identifiers: 4252 ops/sec ±34.14% (0.235ms)
current 128 let and length-100 binding identifiers: 2636 ops/sec ±1.15% (0.379ms)
current 256 let and length-100 binding identifiers: 1318 ops/sec ±1.4% (0.759ms)
current 512 let and length-100 binding identifiers: 658 ops/sec ±0.54% (1.52ms)

This PR also fixed handling of non-BMP and escaped identifiers after let, the following valid cases are now parsed successfully:

let in\u0061;
let in𝐬𝐭𝐚𝐧𝐜𝐞𝐨𝐟;
let 𝐢𝐧;

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