GHA: Test node versions 6,8,10

Test the documented minimum version 6 and some later versions
as was done on Travis.

Using ubuntu-18.04 as I couldn't get these versions to work on newer
ubuntu-20.04 (some horrid c++ compilation errors)

.github/workflows/ci.yml

@@ -64,6 +64,21 @@ jobs:
           VER: '1.17'
         - SWIGLANG: guile
         - SWIGLANG: java
+        - SWIGLANG: javascript
+          ENGINE: node
+          VER: '6'
+          CPPSTD: c++11
+          os: ubuntu-18.04
+        - SWIGLANG: javascript
+          ENGINE: node
+          VER: '8'
+          CPPSTD: c++11
+          os: ubuntu-18.04
+        - SWIGLANG: javascript
+          ENGINE: node
+          VER: '10'
+          CPPSTD: c++11
+          os: ubuntu-18.04
         - SWIGLANG: javascript
           ENGINE: node
           VER: '12'