Support inline $id references

[sinclairzx81]

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

This PR should resolve inline $id references mentioned on issue #410. This implementation adds an additional schemaReferenceMap which is used to collect schemas with $id observed during the build phase. These references are later resolved in subsequent calls to the refFinder(...) function.

Note: There is some overlap here with the existing externalSchema variable which I think may be trying to resolve specifically for anchor (or #) references (I'm entirely not sure). I have left the externalSchema implementation as is as a fall through. It was noted however that the externalSchema variable was undefined for the inline $id / $ref code path.

This PR also adds the $id property to the TS definition for BaseSchema

Submitting for community review

Benchmark

CPU: AMD Ryzen 7 3700X
RAM: 16GB
OS: Windows 10

FJS creation x 7,030 ops/sec ±1.67% (92 runs sampled)
CJS creation x 152,803 ops/sec ±0.20% (94 runs sampled)
AJV Serialize creation x 87,669,032 ops/sec ±0.34% (94 runs sampled)
JSON.stringify array x 3,099 ops/sec ±0.45% (93 runs sampled)
fast-json-stringify array default x 7,551 ops/sec ±0.61% (93 runs sampled)
fast-json-stringify array json-stringify x 7,861 ops/sec ±0.32% (93 runs sampled)
compile-json-stringify array x 7,629 ops/sec ±0.92% (89 runs sampled)
AJV Serialize array x 8,418 ops/sec ±0.64% (95 runs sampled)
JSON.stringify large array x 154 ops/sec ±0.33% (87 runs sampled)
fast-json-stringify large array default x 104 ops/sec ±3.40% (68 runs sampled)
fast-json-stringify large array json-stringify x 153 ops/sec ±0.38% (86 runs sampled)
compile-json-stringify large array x 332 ops/sec ±0.42% (88 runs sampled)
AJV Serialize large array x 96.35 ops/sec ±0.22% (82 runs sampled)
JSON.stringify long string x 9,918 ops/sec ±0.18% (96 runs sampled)
fast-json-stringify long string x 9,991 ops/sec ±0.11% (96 runs sampled)
compile-json-stringify long string x 10,000 ops/sec ±0.09% (97 runs sampled)
AJV Serialize long string x 22,533 ops/sec ±0.72% (97 runs sampled)
JSON.stringify short string x 9,231,883 ops/sec ±0.47% (96 runs sampled)
fast-json-stringify short string x 45,670,735 ops/sec ±0.25% (98 runs sampled)
compile-json-stringify short string x 42,118,334 ops/sec ±0.86% (95 runs sampled)
AJV Serialize short string x 39,980,549 ops/sec ±0.32% (96 runs sampled)
JSON.stringify obj x 1,832,767 ops/sec ±0.74% (93 runs sampled)
fast-json-stringify obj x 11,078,004 ops/sec ±0.45% (97 runs sampled)
compile-json-stringify obj x 21,901,106 ops/sec ±1.34% (95 runs sampled)
AJV Serialize obj x 11,098,735 ops/sec ±1.29% (92 runs sampled)
JSON stringify date x 549,445 ops/sec ±0.23% (91 runs sampled)
fast-json-stringify date format x 898,135 ops/sec ±0.32% (96 runs sampled)
compile-json-stringify date format x 537,925 ops/sec ±0.41% (93 runs sampled)

[mcollina]

lgtm

[sinclairzx81]

@mcollina thanks :)