parser: Add WARNING for integer YAML keys
parser: Add WARNING for integer YAML keys

```bash
benchmark                                               old ns/op     new ns/op     delta
BenchmarkStringifyMapKeysStringsOnlyInterfaceMaps-4     3053          2015          -34.00%
BenchmarkStringifyMapKeysStringsOnlyStringMaps-4        5.23          5.18          -0.96%
BenchmarkStringifyMapKeysIntegers-4                     2320          5177          +123.15%

benchmark                                               old allocs     new allocs     delta
BenchmarkStringifyMapKeysStringsOnlyInterfaceMaps-4     6              6              +0.00%
BenchmarkStringifyMapKeysStringsOnlyStringMaps-4        0              0              +0.00%
BenchmarkStringifyMapKeysIntegers-4                     6              14             +133.33%

benchmark                                               old bytes     new bytes     delta
BenchmarkStringifyMapKeysStringsOnlyInterfaceMaps-4     1008          1008          +0.00%
BenchmarkStringifyMapKeysStringsOnlyStringMaps-4        0             0             +0.00%
BenchmarkStringifyMapKeysIntegers-4                     1008          1776          +76.19%
```
Closes #4393

fedelibre/hugo,mdhender/hugo,gohugoio/hugo,fedelibre/hugo,fedelibre/hugo,mdhender/hugo,mdhender/hugo,anthonyfok/hugo,anthonyfok/hugo,fedelibre/hugo,anthonyfok/hugo,gohugoio/hugo,gohugoio/hugo,mdhender/hugo