feat: add support for raw strings and no interpolation

[techouse]

Adds support for raw strings (i.e r'some string$', r'foo$bar#baz%qux', etc.) to be passed from env files to generated dart files using either:

• the @Envied(rawStrings: true) annotation option which will generate all Strings as raw
• the @EnviedField(rawString: true) annotation option which will generate only the annotated String field as raw

Example:

@Envied(path: 'test/.env.example')
abstract class Env {
  @EnviedField()
  static const String? testString;
  @EnviedField(rawString: true)
  static const String? testUnescapedString;
}

@Envied(path: 'test/.env.example', rawStrings: true)
abstract class Env2 {
  @EnviedField()
  static const String? testString;
  @EnviedField()
  static const String? testUnescapedString;
}

Additionally, I have added the option to disable $BASH_VAR interpolation using either:

• @Envied(interpolate: false)
• @EnviedField(interpolate: false)

It should be used in conjunction with rawString if the user plans on using any chars that might cause throw exceptions, i.e. $.

@EnviedField(
  rawString: true,
  interpolate: false
)
static const String? testUnescapedString; // r'foo$bar%baz|qux#'

---

Fixes #94

[techouse]

@petercinibulk this will require some scrutiny, so please hammer at it once you have time 😎

[petercinibulk]

This looks good to go! Great work as always and thank you! 🙌🏻