Add year-related env vars

Author: matt-dray

.Renviron.example

@@ -12,3 +12,7 @@ NHP_ENCRYPT_KEY=
 NHP_OUTPUTS_URI=https://connect.strategyunitwm.nhs.uk/nhp/{version}/outputs/
 CACHE_VERSION=
 FEEDBACK_FORM_URL=
+YEAR_HORIZON_DEFAULT=2041
+YEAR_HORIZON_MAX=2043
+YEAR_BASELINE_DEFAULT=2024
+YEAR_BASELINE_MIN=2023