`vaultenv` silently fails when writing `MOUNT` in lowercase

[Riscky]

If one creates a secrets file like this:

VERSION 2

mount secret
example#secret

and run vaultenv with this secret file, it will not load any of the secrets defined beyond the mount statement.
This is unexpected behavior, I would expect a parse error (preferably with a suggestion on how to fix the file).
If VERSION is written with lowercase letters, we do get a parse error:

[ERROR] example.secrets:1:8:
  |
1 | version 2
  |        ^
unexpected space
expecting '#' or path component

If any other lowercase string is put on a single line (without # or /), we get the same behavior: any secrets below it are secretly ignored.