Improve camelization for words starting with acronym

[timcbaoth]

These changes want to improve camelization for words that start with capital letters.

When we tried creating a client for Elixir, we would get functions named d_elete_invoice, where we would have expected delete_invoice. The reason being the operationId "DELETE_Invoice" being camelized to dELETEInvoice.

It also applies to words that start with an acronym, like "VATRate" or "ID", which were previously camelized to "vATRate" and "iDxyz", but now should be "vatRate" and "id".

There is also an open issue addressing the unexpected former behaviour: #6559

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[wing328]

@timcbaoth thanks for the PR. Can you please PM me via Slack when you've time?

https://join.slack.com/t/openapi-generator/shared_invite/zt-12jxxd7p2-XUeQM~4pzsU9x~eGLQqX2g

[ghe-bm]

When will this PR be merged into the master? I need this for the jaxrs-spec generator.

[wing328]

will try to get it merge into the next version (7.4.0)

we need some ways for operation Id to fall back to avoid breaking changes. will try to add an option for that.

[wing328]

I've filed #18218 and to enable the new camelization, one will need to set system property openapi.generator.fix.camelize to true