fix: remove duplicate identifier for EnvVarError

[avocadomaster]

#150 to avoid ts error. EnvVarError class is exported on line 263

../../env-var/env-var.d.ts:263:14 - error TS2300: Duplicate identifier 'EnvVarError'.

263 export class EnvVarError extends Error {}
                 ~~~~~~~~~~~

../../env-var/env-var.d.ts:308:14 - error TS2300: Duplicate identifier 'EnvVarError'.

308 export const EnvVarError: EnvVarError
                 ~~~~~~~~~~~


Found 2 errors.

[coveralls]

Coverage remained the same at 100.0% when pulling 355e6c0 on avocadomaster:master into 03f4fc2 on evanshortiss:master.

[evanshortiss]

Thank you @avocadomaster. I'm not sure why this wasn't caught by tests. I'll really need to look into better TS typing tests.

[evanshortiss]

Issues like this will now be caught due to 47195ca