Add proposal for upper case mapping

[patroza]

Motivation

Currently there is no conversion done (except nesting . -> _), and the rest of the format for Flat or Map is the same as for environment variables.
Which seems awkward to me.

Now if I define a Config, and want to read from Env, I am kind of forced to use ugly upper-case key names in my Config, making the Config specialised for Environment variables.
Or I must break with convention and live with camelCase key names to fit with Flat or Map providers.

Proposal

Auto convert from/to Environment variable convention.

The convention for environment variables classically is UPPERCASE,
word separation is clasically with _ so: HOST_PORT.
Nesting is currently also done with _, so I would instead propose __ to avoid reasonable colissions.

key - current - suggestion

• host - host -> HOST
• hostPort - hostPort -> HOST_PORT
• hostPort.port - hostPort_port -> HOST_PORT__PORT

TODO

Auto replace any unsupported symbol with __?
hubla@abc% -> HUBLA__ABC__

sample of invalid:

> export ABC.ABC=1
export: not valid in this context: ABC.ABC
> export "ABC.ABC"=1
export: not valid in this context: ABC.ABC
> export ABC\.ABC=1
export: not valid in this context: ABC.ABC

This might however prove challenging because apparently there's also a return conversion :)

Alternative

Provide the ability to provide a transformer to a ConfigProvider, so that one may override it with their own convention.
Extending

  export interface FromEnvConfig {
    readonly pathDelim: string
    readonly seqDelim: string
    readonly conversion: (key: string) => string
    readonly reverseConversion: (key: string) => string
  }

[patroza]

Reported upstream at zio/zio#7727

[datner]

Is adding dependencies for this really necessary? This is not something hard to implement without increasing the bundle size. Some regex magic can absolutely do this. Maybe something like /([A-Z]?[a-zA-Z0-9]+)/g you can then iterate over then something like

key.replace('.','_').replace(regexFromBefore, (m, ...args) => {
  const hasNamedGroups = typeof args.at(-1) === "object"
  const offset = hasNamedGroups ? args.at(-3) : args.at(-2)
  const spacer = offset === 0 ? '' : '_'
  return m + spacer
}) 

This can be changed to something more robust. Either way, no deps needed

[patroza]

@datner this isn't adding dependencies.
also the change will be a different one: zio/zio#7731