Port to Babel 7

[bazineta]

Use the Babel 7 @babel/core transpiler, falling back to the older babel-core version if possible.

[rdeforest]

Cakefile needs some love too I think?

@@ -67,7 +67,11 @@ build = (callback) ->
   buildExceptParser callback

 transpile = (code) ->
-  babel = require 'babel-core'
+  try
+    babel = require '@babel/core'
+  catch
+    babel = require 'babel-core'
+
   presets = []
   # Exclude the `modules` plugin in order to not break the `}(this));`
   # at the end of the `build:browser` code block.

[GeoffreyBooth]

Cakefile needs some love too I think?

Cakefile is only run by someone who’s checked out the CoffeeScript repo, and therefore will have whatever dependencies and devDependencies are defined in CoffeeScript’s package.json. We should update CoffeeScript itself to use Babel 7 and @babel/core, and then also update Cakefile accordingly. We don’t need to support either babel-core or @babel/core in the Cakefile, though, as either one or the other will be specified in package.json.