Support for multiline environment values

The multiline values could be a good addon to `runenv` features.

To support multiline 	we can utlize different quotes. Possible options I'm thinking of are:

- ''  ''      - doubled single quotes - like in nix
- `  `       - single backtick like in dotenvx, 
- ``` ```    - triple backticks like in slack, discord, markdown
- """ """ - triple double quotes like in python