Rename the new JSON and ION expressions to less technical names (decode and encode can be confusing)

[anna-geller]

Feature description

I think we've made a mistake here #4289 with the naming jsonEncode + jsonDecode + ionEncode and ionDecode - they seem too technical. (@loicmathieu you didn't do anything wrong by following the issue, mistake on my end with the too-technical naming)

Let's rename them to friendlier names before the release.

I think it's worth renaming as follows:

1. jsonEncode --> toJson = Serializes a data structure into a JSON string
2. jsonDecode --> fromJson = Deserializes a JSON string into a data structure
3. ionEncode --> toIon = Serializes a data structure into an ION string
4. ionDecode --> fromIon = Deserializes an ION string into a data structure

pseudo-code:

json_string = toJson(data)
data = fromJson(json_string) 

pebble:

{{ [1, 2, 3] | toJson  }} # "[1,2,3]"
{{ fromJson( '{"name": "kestra", "year": 2024}' ).name }} # kestra

After we rename, I'll update the Breaking Changes notes

[loicmathieu]

We don't have ionEncode filter, it was not listed in the original issue: #3715
@anna-geller Should I add it ?

[anna-geller]

yes, let's add it for consistency 👍