app does not sanitize invalid identifiers

[3052]

valid input:

{"1": "hello", "2": "world"}

invalid output:

syntax = "proto3";

message SomeMessage {
    string 1 = 1;
    string 2 = 2;
}

https://protobuf.dev/reference/protobuf/proto3-spec/#identifiers

[AlexL70]

Just opened a PR that is supposed to close this issue: #10