Readd tar block. Fixes #103

The proper way to resolve the redundancy between `building_blocks:Tar` and `moreblocks:tar` is with an alias in building_blocks as noted in issue #68.

The recipe conflict with streetsmod has been resolved by using `default:pine_tree` in place of `default:gravel`.  This may also be more realistic than gravel as a source of tar because pine derived tar was once of major economic importance.

nodes.lua

@@ -113,6 +113,12 @@ local nodes = {
 		is_ground_content = false,
 		sounds = sound_stone,
 	},
+	["tar"] = {
+		description = S("Tar"),
+		groups = {cracky=2, tar_block=1},
+		is_ground_content = false,
+		sounds = sound_stone,
+	},
 	["dirt_compressed"] = {
 		description = S("Compressed Dirt"),
 		groups = {crumbly=2},