feat(plugin): add terrain3d plugin installation

addons/terrain_3d/menu/channel_packer_import_template.txt

@@ -0,0 +1,32 @@
+[remap]
+
+importer="texture"
+type="CompressedTexture2D"
+metadata={
+"imported_formats": ["s3tc_bptc"],
+"vram_texture": true
+}
+
+[deps]
+
+source_file="$SOURCE_FILE"
+
+[params]
+
+compress/mode=2
+compress/high_quality=$HIGH_QUALITY
+compress/lossy_quality=0.7
+compress/hdr_compression=1
+compress/normal_map=2
+compress/channel_pack=0
+mipmaps/generate=$GENERATE_MIPMAPS
+mipmaps/limit=-1
+roughness/mode=0
+roughness/src_normal=""
+process/fix_alpha_border=true
+process/premult_alpha=false
+process/normal_map_invert_y=false
+process/hdr_as_srgb=false
+process/hdr_clamp_exposure=false
+process/size_limit=0
+detect_3d/compress_to=1