json 'dumps' filter for creating javascript with mako

Migrated issue, originally created by jvanasco (@jvanasco)

I ran into some issues with Mako when trying to make javascript using the output of json.dumps

# in python
python_dict = { "hello" : """This is an example object I am using some 'single quotes' and "double quotes".""", }

usually i can just disable filtering in mako :

// in js template 
var payload = '${simplejson.dumps(python_dict)|n}';

unfortunately in this case the single-quotes break. we get this

// in js generated
var payload = '{"hello": "This is an example object I am using some 'single quotes' and \"double quotes\"."}'

this appears to work fine :

// in js template 
var payload = '${simplejson.dumps(python_dict).encode('string-escape')|n}';

which generates

// in js generated
var payload = {"hello": "This is an example object I am using some \'single quotes\' and \\"double quotes\\"."}

there are a handful of miscellaneous JS+Mako questions floating on StackOverflow and blogs... so i think it might be worthwhile to have an official json escaping method in mako templates.

The issue names no files or tests. Start by locating Mako's existing filtering and escaping entry points and related template-output tests; use the examples to define the expected JSON-to-JavaScript escaping behavior, then add coverage showing that both single and double quotes remain valid.