fix: parse MAILDEV_HTTPS as a boolean (#294)

MAILDEV_HTTPS is listed in BOOLEAN_VARS, but envVarToConfigKey() had no
entry for `https`, so the boolean pass skipped it and the raw string from
ENV_MAPPING stayed in the config. MAILDEV_HTTPS=false then enabled HTTPS
and validation demanded httpsCert and httpsKey.

.changeset/fix-https-env-boolean.md

@@ -0,0 +1,7 @@
+---
+'maildev': patch
+---
+
+Parse `MAILDEV_HTTPS` as a boolean (#294). It was passed through as the raw
+string, so `MAILDEV_HTTPS=false` still enabled HTTPS and startup failed asking
+for `httpsCert` and `httpsKey`.