Creates a short dummy variable name from a set name following conventions:
Uses preferred names for common sets (tech -> h, slice -> ts, etc.)
First letter of the set name (lowercase) for others
If it's an alias, use two letters (checking against all_set_names)
Ensures uniqueness by checking against all symbols in model
Examples
generate_index_alias("tech") # "h"
#> [1] "h"
generate_index_alias("slice") # "ts"
#> [1] "ts"
generate_index_alias("commp", all_set_names = c("comm", "commp")) # "cp"
#> [1] "cp"
