feat: add full monorepo support

[chihab]

Given:

/home/user/monorepo
 apps
    frontend1
       .env
    frontend2
       .env.dev
       .env
.env
.env.dev

and the configuration below in frontends' angular.json:

"ngxEnv": {
  "prefix": "NGX_",
  "path": "../../"
},

Consumed dot env files will be ordered by hierarchy as follows:
frontend1

/home/user/monorepo/apps/frontend1/.env
/home/user/monorepo/.env.dev
/home/user/monorepo/.env

frontend2

/home/user/monorepo/apps/frontend2/.env.dev
/home/user/monorepo/apps/frontend2/.env
/home/user/monorepo/.env.dev
/home/user/monorepo/.env

ngxEnv configuration
These paths are equivalent:
"path": "../../"
"path": "../../.env"
"path": "/home/user/monorepo"
"path": "/home/user/monorepo/.env"

[chihab]

@otonielguajardo it would be great if you could take a look at this PR to follow up on our discussion here #43 :)

[otonielguajardo]

This looks awesome @chihab,

This will make it much easier to work with global environment variables in monorepos that include Angular applications.