[all] BUG: intentional underscore in front of fields getting removed on camelize

Description

Currently if we have an spec where one field (e.g. _links) is defined the current StringUtils replaces the first underscore on camelize. This doesn't seem right so I fixed it in my branch. However there are couple of Tests that fail then:
org/openapitools/codegen/go/GoModelTest.java:282
org/openapitools/codegen/php/PhpModelTest.java:284
and so on.
The correct behavior should be IMHO {"_sample", "_Sample"}, instead of {"_sample", "Sample"},

openapi-generator version

master HEAD 774013c7

OpenAPI declaration file content or url

https://gist.github.com/sruehl/18b8b704586ad7b39e488018c17ef7e5

Command line used for generation

generate -i modules/openapi-generator/src/test/resources/2_0/petstore.yaml -g typescript-axios -o samples/client/petstore/typescript-axios/builds/default $@

Steps to reproduce

1. Generate
2. look at api.ts and see that _links has become links and therefore is undefined at runtime.

Related issues/PRs

#1634

Suggest a fix/enhancement

keep the first underscore on camelCase

Start by locating StringUtils and its camelize behavior, then run the mentioned org/openapitools/codegen/go/GoModelTest.java and org/openapitools/codegen/php/PhpModelTest.java tests. Confirm that leading underscores such as _sample remain in generated names while the expected camelized form is _Sample, and update affected expectations consistently.