snap: Quote "@babel/cli" to solve build error

  Cannot parse snapcraft.yaml from ~gohugoio/hugo/+git/hugo:master:
  while scanning for the next token found character ‘@’ that cannot
  start any token in “”, line 70, column 34: node-packages:
  [postcss-cli, @babel/cli] ^

Also:

- Upgrade to snapcraft 3.x by adding "base: core"
- Add "license: Apache-2.0"

snap/snapcraft.yaml

@@ -1,4 +1,5 @@
 name: hugo
+base: core
 version: "0.71.0-DEV"
 summary: Fast and Flexible Static Site Generator
 description: |
@@ -7,6 +8,7 @@ description: |
   with content and templates and renders them into a full HTML website.
 confinement: strict
 grade: devel # "devel" or "stable"
+license: Apache-2.0
 
 apps:
   hugo:
@@ -67,7 +69,9 @@ parts:
 
   node:
     plugin: nodejs
-    node-packages: [postcss-cli, @babel/cli]
+    node-packages:
+        - postcss-cli
+        - "@babel/cli"
     filesets:
       node:
         - bin/node