Fork samples so Lit 3 docs samples use Lit 3 pre-release (#1127)

Lit 3 pre-release docs now include samples that are using the Lit 3 pre-release, forked from v2 samples.

Author: AndrewJakubowicz

packages/lit-dev-content/samples/base.json

@@ -1,8 +1,3 @@
 {
-  "files": {
-    "package.json": {
-      "content": "{\n  \"dependencies\": {\n    \"lit\": \"^2.0.0\",\n    \"@lit/reactive-element\": \"^1.0.0\",\n    \"lit-element\": \"^3.0.0\",\n    \"lit-html\": \"^2.0.0\"\n  }\n}",
-      "hidden": true
-    }
-  }
+  "extends": "/samples/v2-base.json"
 }

packages/lit-dev-content/samples/tsconfig.json

@@ -19,10 +19,13 @@
     "examples/hello-world/simple-greeting.ts",
     // Comments in some locations get lost by TypeScript. See
     // https://github.com/lit/lit/issues/2158.
-    "docs/templates/lists-arrays/my-element.ts",
-    "docs/templates/lists-map/my-element.ts",
+    "v2-docs/templates/lists-arrays/my-element.ts",
+    "v2-docs/templates/lists-map/my-element.ts",
+    "v3-docs/templates/lists-arrays/my-element.ts",
+    "v3-docs/templates/lists-map/my-element.ts",
     // Mixin sample does not compile: https://github.com/lit/lit/issues/3865
-    "docs/mixins/highlightable/**",
+    "v2-docs/mixins/highlightable/**",
+    "v3-docs/mixins/highlightable/**",
     // Intentionally doesn't compile for tutorial part.
     "tutorials/word-viewer/02/before/word-viewer.ts"
   ]

packages/lit-dev-content/samples/v2-base.json

@@ -0,0 +1,8 @@
+{
+  "files": {
+    "package.json": {
+      "content": "{\n  \"dependencies\": {\n    \"lit\": \"^2.0.0\",\n    \"@lit/reactive-element\": \"^1.0.0\",\n    \"lit-element\": \"^3.0.0\",\n    \"lit-html\": \"^2.0.0\"\n  }\n}",
+      "hidden": true
+    }
+  }
+}

packages/lit-dev-content/samples/docs/components/events/child/index.html renamed to packages/lit-dev-content/samples/v2-docs/components/events/child/index.html

@@ -1,5 +1,5 @@
 {
-  "extends": "/samples/base.json",
+  "extends": "/samples/v2-base.json",
   "files": {
     "my-element.ts": {},
     "index.html": {}

packages/lit-dev-content/samples/docs/components/events/child/my-element.ts renamed to packages/lit-dev-content/samples/v2-docs/components/events/child/my-element.ts

@@ -1,5 +1,5 @@
 {
-  "extends": "/samples/base.json",
+  "extends": "/samples/v2-base.json",
   "files": {
     "my-dispatcher.ts": {},
     "my-listener.ts": {},