Improve support for ION/JSON processing in Pebble expressions

[anna-geller]

Required todos:

• rename the json filter to jsonEncode for more clarity about what it does — it takes a data structure or object and converts it into a JSON string e.g. {{ [1, 2, 3] | jsonEncode }}
• rename the json() function to jsonDecode() to make it clear that it takes a JSON string and converts it into a data structure or object so the user can access further attributes using a dot notation e.g. '{{ jsonDecode(read(taskrun.items)) }}'
• add a new ionDecode() function that will take an ION string (from reading an ION file) and convert it into a data structure or object e.g. '{{ ionDecode(read(taskrun.items)) }}'; this function can only process a single row

Important notes:

• The renaming should ideally be implemented in a non-breaking way e.g. using json() will raise a warning in the UI but it will still work
• The goal of these tasks is to make it easy to process data split by ForEachItem. However, the ionDecode() function should raise an error when trying to parse a multi-line string (i.e. ION file with multiple rows); this is important for performance reasons