flake: emit Unity bundle zip; add Gitea Actions CI/release; publish VPM index

- flake.nix: install the libaac cdylib alongside aac-dump and zip it with
  the C# Unity packages as the default package
- .gitea/workflows: build+package on push/PR, publish release on v* tags
- vpm: commit the repository index and packaging script, with VPM_INDEX and
  VPM_INDEX_URL overrides for generating a local listing

vpm/index.json

@@ -0,0 +1,30 @@
+{
+  "name": "Doloro VPM Packages",
+  "id": "dev.doloro.vpm",
+  "url": "https://git.scug.io/doloro/av3-animation-as-crab/-/raw/main/vpm/index.json",
+  "author": {
+    "name": "doloro"
+  },
+  "packages": {
+    "dev.doloro.animator-as-crab": {
+      "versions": {
+        "0.1.0": {
+          "name": "dev.doloro.animator-as-crab",
+          "displayName": "Animator As Crab",
+          "version": "0.1.0",
+          "unity": "2019.4",
+          "description": "Writes Animator Controllers from a Rhai script, evaluated by the libaac native library.",
+          "url": "https://git.scug.io/doloro/av3-animation-as-crab/-/releases/download/v0.1.0/dev.doloro.animator-as-crab-0.1.0.zip",
+          "zipSHA256": "e0ad8c5717059115567e5c7907425d4a6a1ce1613023197061796d0b5128a515",
+          "dependencies": {
+            "com.unity.nuget.newtonsoft-json": "3.2.1"
+          },
+          "vpmDependencies": {},
+          "author": {
+            "name": "doloro"
+          }
+        }
+      }
+    }
+  }
+}