feat: _ can be used as separators in environment variable names

[kenjis]

Description
You can use both as environment variable name:

• database.default.hostname (now) - looking for it first
• database_default_hostname (new) - if not found, looking for it

See #4026

Checklist:

• Securely signed commits
• Component(s) with PHPdocs
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[MGatner]

The code looks good. Could you share your motivation behind this change? Are there other frameworks that use underscores? Or perhaps deployment solutions? Or do you have an underscore fetish? 😉

[samsonasik]

never use _ usage, what if user has existing _ config as a part of key value?

SimpleConfig.onedeep_value=baz

as onedeep_value as the key itself. is that previously working or after this change, that will still works?

[kenjis]

@MGatner Some environments can't use environment variables with dot.
See #4026

And I saw this tweet:
https://twitter.com/eclairtom/status/1443603184597962752
English Translation:

Versioned up to #Docker desctop 4.1.0. docker-compose is no longer usable.
The .env used by CodeIgniter4 is now affected. Because it uses constants that contain dots. (4.0 ignored.)
I have to change the folder structure for docker at worst.

[kenjis]

@samsonasik Good question!
I think property name like onedeep_value is not used by CI4,
but users might use it.

I added a test case, and it passed.

[michalsn]

I think this is probably the right direction to solve the problem with dot variables for some environments.

The only thing missing is documentation. We don't want this to become a secret feature, so it needs to be documented in the user guide.

[kenjis]

@michalsn Thanks! I added it to the documentation.

[MGatner]

Nice teamwork! This looks ready to me.