fix extension typing error

[ChibiBlasphem]

When creating extraAccessors there's a typing problem:

I'm not fluent in english so i'll illustrate with an example:

const env = from(process.env, {
  asEnv: (value: string): 'development' | 'production' | 'test' => {
    if (value !== 'production' && value !== 'production' && value !== 'test') {
      throw new Error('must be "production", "development", "test"')
    }
    return value
  },
})

When using it I have an error:

const nodeEnv = env.get('NODE_ENV').default('production').asEnv();

Typescript complains about "value" argument not being specified

I read the typings and even if I use required

const nodeEnv = env.get('NODE_ENV').default('production').required().asEnv();

TS does not complain anymore, but the type of params is not correctly infered from the extraAccessor definition

[coveralls]

Coverage remained the same at 100.0% when pulling d87d8e9 on ChibiBlasphem:fix-extension-typing-error into 57fcded on evanshortiss:6.x.

[evanshortiss]

Great find @ChibiBlasphem.

It's strange that the example included in the repo doesn't have this issue - I haven't investigated why.

I've changed your PR to target the 6.x branch so it can be included in the 6.2.0 release. There's some conflicts, could you take a look?

[evanshortiss]

Thanks @ChibiBlasphem