feat: add constant-case strategy in building env key names

[mojixcoder]

Closes #32

This is just a minimal implementation to see if you're ok with the way I added this new feature or not.
More test cases can be added.

[kkyr]

With input = "helloWORLD" this algorithm will output "helloWORLD". Should it not output "hello_WORLD"?

[mojixcoder]

This should be helloWorld, I did this to avoid converting USA to u_s_a.
Maybe we should rename this to camelCaseToConstantCase?

[kkyr]

Why would "helloWORLD" -> "hello_WORLD" imply "USA" -> "u_s_a"?

[mojixcoder]

If next letter of an upper case letter is lower case, it means it has to be separated with an underline. helloWorld -> hello_World.

if an upper case letter is preceded by another upper case letter, it means it's something like USA.

[mojixcoder]

Current behavior only checks next letters each letter, that's why we can convert something like USACountryCode to USA_Country_Code but we can't convert helloWORLD to hello_WORLD.

I'm gonna think about another algorithm.

[mojixcoder]

I updated the algorithm and added more test cases, please check if that's what you meant.

[kkyr]

Algorithm seems to work but is a little hard to read - what do you think about this version?

func (f *fig) toConstantCase(key string) string {
	var b strings.Builder
	runes := []rune(key)

	lowerToUpper := func(i int) bool {
		return i > 0 && unicode.IsLower(runes[i-1]) && unicode.IsUpper(runes[i])
	}
	upperToLower := func(i int) bool {
		return i+1 < len(runes) && unicode.IsUpper(runes[i]) && unicode.IsLower(runes[i+1])
	}

	for i := range runes {
		if i > 0 && (lowerToUpper(i) || upperToLower(i)) {
			b.WriteByte('_')
		}
		b.WriteRune(runes[i])
	}
	
	return strings.ToUpper(b.String())
}

[kkyr]

This is called constant case but it's not upper casing the output.

[mojixcoder]

strings.Upper is called in its parent function.
Maybe we should put the default behaviour in another strategy and be able to completely override the envKeyTransformer by providing another transformer, right?

[kkyr]

Yes

[kkyr]

We can introduce an abstraction here so that multiple strategies can coexist without cluttering this struct and formatEnvKey with each option:

func defaultFig() *fig {
	return &fig{
		envKeyTransform: strings.ToUpper
	}
}

type fig struct {
    envKeyTransform func(string) string
}

...

func (f *fig) formatEnvKey(key string) string {
   ..
   key = f.envKeyTransform(key)
}

...

func UseConstantCaseForEnvKeys() Option {
	return func(f *fig) {
		f. envKeyTransform = toConstantCase
	}
}

[mojixcoder]

Should we pass the env prefix to transformers? it's needed for adding prefix to the generated env key.

[kkyr]

Yeah we should add it