DotEnvSource
Reads a .env file of KEY=value lines, with keys in SCREAMING_SNAKE_CASE. Unlike the other file sources, keys are matched exactly, including case.
The parser supports blank lines, full-line # comments, a leading export , and # comments after unquoted values. Double-quoted values unescape \n, \r, \t, \" and \\; single-quoted values are taken literally. Lines without = are skipped, and a later duplicate key wins. Multiline values and ${VAR} expansion are not supported.