Separate the concept of declaration and definition

Add a new #declare command. #define and #register should be treated as an alias of #declare. All resources can be declared (#597).

A CacheUnit should now have dcl, def, doc, and ref fields.

Here's a table showing the possible ways to declare/define each type of resource.

Type	Declaration	Definition
Advancement	`#declare advancement <ID>`	An advancement file.
Biome	`#declare worldgen/biome <ID>`	A biome file.
Block Tag	`#declare tag/block <ID>`	A block tag file.
Bossbar	`#declare bossbar <ID>`	`bossbar add <ID>`
Configured Carver	`#declare worldgen/configured_carver <ID>`	A configured carver file.
Configured Feature	`#declare worldgen/configured_feature <ID>`	A configured feature file.
Configured Structure Feature	`#declare worldgen/configured_structure_feature <ID>`	A configured structure feature file.
Configured Surface Builder	`#declare worldgen/configured_surface_builder <ID>`	A configured surface builder file.
Dimension	`#declare dimension <ID>`	A dimension file.
Dimension Type	`#declare dimension_type <ID>`	A dimension type file.
Entity	`#declare entity <ID>`	N/A
Entity Type Tag	`#declare tag/entity_type <ID>`	An entity type tag file.
Fluid Tag	`#declare tag/fluid <ID>`	A fluid tag file.
Function	`#declare function <ID>`	A function file.
Function Tag	`#declare tag/function <ID>`	A function tag file.
Item Tag	`#declare tag/item <ID>`	An item tag file.
Loot Table	`#declare loot_table <ID>`	A loot table file.
Objective	`#declare objective <ID>`	`scoreboard objectives add <ID>`
Predicate	`#declare predicate <ID>`	A predicate file.
Processor List	`#declare worldgen/processor_list <ID>`	A processor list file.
Recipe	`#declare recipe <ID>`	A recipe file.
Score Holder	`#declare score_holder <ID>`	N/A
Storage	`#declare storage <ID>`	N/A
Tag	`#declare tag <ID>`	N/A
Team	`#declare team <ID>`	N/A
Template Pool	`#declare worldgen/template_pool <ID>`	A template pool file.

SpyglassMC / Spyglass

Separate the concept of declaration and definition #623