support defaults functions

[x] default: Give a default value. Used like this: trim " "| default "empty". Since trim produces an empty string, the default value is returned. For things with a length (strings, slices, maps), len(0) will trigger the default. For numbers, the value 0 will trigger the default. For booleans, false will trigger the default. For structs, the default is never returned (there is no clear empty condition). For everything else, nil value triggers a default.
[ ] empty: Return true if the given value is the zero value for its type. Caveats: structs are always non- [ ]empty. This should match the behavior of {{if pipeline}}, but can be used inside of a pipeline.
[ ] coalesce: Given a list of items, return the first non-empty one. This follows the same rules as 'empty'. '{{ coalesce .someVal 0 "hello" }} will return.someVal` if set, or else return "hello". The 0 is skipped because it is an empty value.
[ ] compact: Return a copy of a list with all of the empty values removed. 'list 0 1 2 "" | compact' will return '[1 2]'

fiji-flo / sprig-rust