Re-add global env variable substitution

[Envek]

Re-add behavior that was first introduced in #54 and then removed in #58.

This behavior is present in Ruby dotenv (see spec/dotenv/parser_spec.rb#L66-L69)

This pull request is basically a rebase of #72 on top of current main branch (with parser rewritten).

See #74 for discussion.

Closes #72 Closes #74 Closes #88 Closes #102 Closes #103 Closes #135 Closes #149