BigInt: additional info frontmatter. Validated features (#1518)

test/built-ins/BigInt/asIntN/bigint-tobigint-errors.js

test/built-ins/BigInt/asIntN/bigint-tobigint-toprimitive.js

...built-ins/BigInt/asIntN/bigint-tobigint-wrapped-values.js

test/built-ins/BigInt/asIntN/bits-toindex-errors.js

test/built-ins/BigInt/asIntN/bits-toindex-toprimitive.js

test/built-ins/BigInt/asIntN/bits-toindex-wrapped-values.js

test/built-ins/BigInt/asUintN/bigint-tobigint-errors.js

test/built-ins/BigInt/asUintN/bigint-tobigint-toprimitive.js

...uilt-ins/BigInt/asUintN/bigint-tobigint-wrapped-values.js

test/built-ins/BigInt/asUintN/bits-toindex-errors.js

test/built-ins/BigInt/asUintN/bits-toindex-toprimitive.js

test/built-ins/BigInt/asUintN/bits-toindex-wrapped-values.js

test/built-ins/BigInt/constructor-empty-string.js

test/built-ins/BigInt/constructor-from-binary-string.js

test/built-ins/BigInt/constructor-from-decimal-string.js

test/built-ins/BigInt/constructor-from-hex-string.js

test/built-ins/BigInt/constructor-from-octal-string.js

...built-ins/BigInt/constructor-from-string-syntax-errors.js

test/built-ins/BigInt/constructor-integer.js

test/built-ins/BigInt/constructor-trailing-leading-spaces.js