stdlib: add support for util.inspect.custom to AppError

[dirkdev98]

This is used when AppErrors are passed to Node.js' util#inspect directly, or indirectly via console.log.
It does not work with console.dir tho.

Closes #453

[github-actions]

==== Benchmark result ====

Node.js version: 14

Total benchmarks: 9
          Passed: 9
          Failed: 0
-----------
object validator simple      5000000  iterations     485  ns/op
object validator nested        50000  iterations   47141  ns/op
logger - strings              500000  iterations    2320  ns/op
logger - objects              500000  iterations    2814  ns/op
logger - deep objects         500000  iterations    4789  ns/op
uuid                         2000000  iterations     563  ns/op
uuid.isValid                10000000  iterations     165  ns/op
isNil                      500000000  iterations       0  ns/op
isPlainObject              100000000  iterations      19  ns/op

[github-actions]

==== Benchmark result ====

Node.js version: 15

Total benchmarks: 9
          Passed: 9
          Failed: 0
-----------
object validator simple      5000000  iterations     467  ns/op
object validator nested        50000  iterations   45989  ns/op
logger - strings              500000  iterations    2067  ns/op
logger - objects              500000  iterations    2489  ns/op
logger - deep objects         500000  iterations    4374  ns/op
uuid                         2000000  iterations     611  ns/op
uuid.isValid                10000000  iterations     128  ns/op
isNil                      500000000  iterations       0  ns/op
isPlainObject              100000000  iterations      17  ns/op

[github-actions]

==== C8 test coverage ====
Statements: 70.79% ( 17969/25384 )
Branches : 71.62% ( 2110 /2946 )
Functions : 70.69% ( 668 /945 )
Lines : 70.79% ( 17969 /25384 )