code-gen: only use an extensions in imports in ES Modules

[dirkdev98]

TypeScript doesn't rewrite paths, so when the target is not ES modules, we should not specify import extensions. The easiest way is to use type: "module" from the package.json

[github-actions]

==== Benchmark result ====

Node.js version: 14

Total benchmarks: 13
          Passed: 13
          Failed: 0
-----------
lbu validator simple                    2000000  iterations      537  ns/op
yup validator simple                      20000  iterations    62797  ns/op
fastest-validator validator simple     10000000  iterations      136  ns/op
lbu validator nested                      50000  iterations    47143  ns/op
yup validator nested                       2000  iterations   502817  ns/op
fastest-validator validator nested      2000000  iterations      950  ns/op
logger - strings                         500000  iterations     2447  ns/op
logger - objects                         500000  iterations     2989  ns/op
logger - deep objects                    200000  iterations     5406  ns/op
uuid                                    2000000  iterations      556  ns/op
uuid.isValid                           10000000  iterations      170  ns/op
isNil                                 500000000  iterations        0  ns/op
isPlainObject                         100000000  iterations       19  ns/op

[github-actions]

==== C8 test coverage ====
Statements: 70.39% ( 18041/25630 )
Branches : 71.52% ( 2114 /2956 )
Functions : 70.75% ( 670 /947 )
Lines : 70.39% ( 18041 /25630 )

[github-actions]

==== Benchmark result ====

Node.js version: 15

Total benchmarks: 13
          Passed: 13
          Failed: 0
-----------
lbu validator simple                    5000000  iterations      495  ns/op
yup validator simple                      20000  iterations    76194  ns/op
fastest-validator validator simple     10000000  iterations      121  ns/op
lbu validator nested                      50000  iterations    50013  ns/op
yup validator nested                       2000  iterations   603077  ns/op
fastest-validator validator nested      2000000  iterations      864  ns/op
logger - strings                         500000  iterations     2362  ns/op
logger - objects                         500000  iterations     2943  ns/op
logger - deep objects                    200000  iterations     5014  ns/op
uuid                                    2000000  iterations      665  ns/op
uuid.isValid                           10000000  iterations      149  ns/op
isNil                                 500000000  iterations        0  ns/op
isPlainObject                         100000000  iterations       19  ns/op