[Nim] Fix compilation in case of schema with enum constraint with an enum value being not a valid nim identifier

[sandwoodK]

This is a fix for bug #20779

In case of a schema with an enum constraint, nim code generation creates a nim objtect type enum with the enum values of this schema enum.
It must be valid identifier otherwise the code can not compile.
The fix is to
1/ detect if it is a valid nim identifier, job done with the isValidIdentifier private function, using a pattern based on the definition of a nim valid identifier (see https://nim-lang.org/docs/manual.html#lexical-analysis-identifiers-amp-keywords)
2/ If it is not a valid identifier, we surround the identifier with backticks.

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@hokamoto

[sandwoodK]

Note : I have identified others cases where generated Nim code will not compile due to invalid identifiers, I prefer to have atomic PRs instead of a huge 'fix them all'.

I also need guidance of how to add unit tests for these edge cases so that the behaviour changes on generated code becomes more readeable when reviewing..

[wing328]

thanks for the PR

please pm me via slack to quickly walk through adding a test

https://join.slack.com/t/openapi-generator/shared_invite/zt-2wmkn4s8g-n19PJ99Y6Vei74WMUIehQA

[wing328]

i just merged #20819 into master

can you please pull the latest master into your branch?

and then update the test spec (modules/openapi-generator/src/test/resources/3_0/nim/petstore.yaml) with test schemas/endpoints.