perf(marshal): Replace more XS-expensive string operations

[gibson042]

Most notably, prefer charAt(0) === ch over startsWith(ch).

$ esbench --eshost-option '-h *XS' --init '
const c50 = "I am the *very* model of a modern major general.\n\n";
const c500 = Array(10).fill(c50).join("");
const c5k = Array(100).fill(c50).join("");
const c50k = Array(1000).fill(c50).join("");
' '' '{                                     
  "c50.startsWith(ch)": `result = c50.startsWith("I");`,
  "c500.startsWith(ch)": `result = c500.startsWith("I");`,
  "c5k.startsWith(ch)": `result = c5k.startsWith("I");`,
  "c50k.startsWith(ch)": `result = c50k.startsWith("I");`,
  "c50[0]": `result = c50[0] === "I";`,
  "c500[0]": `result = c500[0] === "I";`,
  "c5k[0]": `result = c5k[0] === "I";`,
  "c50k[0]": `result = c50k[0] === "I";`,
  "c50.charAt(0)": `result = c50.charAt(0) === "I";`,
  "c500.charAt(0)": `result = c500.charAt(0) === "I";`,
  "c5k.charAt(0)": `result = c5k.charAt(0) === "I";`,
  "c50k.charAt(0)": `result = c50k.charAt(0) === "I";`,
}'
#### Moddable XS
c50.startsWith(ch): 9.17 ops/ms
c500.startsWith(ch): 5.03 ops/ms
c5k.startsWith(ch): 1.84 ops/ms
c50k.startsWith(ch): 0.22 ops/ms
c50[0]: 8.20 ops/ms
c500[0]: 6.06 ops/ms
c5k[0]: 1.85 ops/ms
c50k[0]: 0.24 ops/ms
c50.charAt(0): 8.55 ops/ms
c500.charAt(0): 8.06 ops/ms
c5k.charAt(0): 9.09 ops/ms
c50k.charAt(0): 8.70 ops/ms

Ref #1982
Ref #2001

Security Considerations

n/a

Scaling Considerations

As stated on the tin.

Documentation Considerations

n/a

Testing Considerations

n/a

Compatibility Considerations

n/a

Upgrade Considerations

Some error messages are affected.

[erights]

LGTM, thanks!

[erights]

Good. Consolidating the regexp makes this much more declarative and readable. Nice!

[erights]

Please include enough info so others would know how to rerun your timing tests.