Add support for nested JSON syntax

[BoboTiG]

Proposal

Data fields (key=value) and raw JSON fields (key:=value) now parse the key as JSON form.

Example:

$ http PUT pie.dev/put \
    'object=scalar' \                           # Object — blank key
    'object[0]=array 1' \                       # Object — "0" key
    'object[key]=key key' \                     # Object — "key" key
    'array:=1' \                                # Array — first item
    'array:=2' \                                # Array — second item
    'array[]:=3' \                              # Array — append (third item)
    'wow[such][deep][3][much][power][!]=Amaze'  # Nested object

{
    "array": [
        1,
        2,
        3
    ],
    "object": {
        "": "scalar",
        "0": "array 1",
        "key": "key key"
    },
    "wow": {
        "such": {
            "deep": [
                null,
                null,
                null,
                {
                    "much": {
                        "power": {
                            "!": "Amaze"
                        }
                    }
                }
            ]
        }
    }
}

The new syntax is compatible with other JSON-specific syntax and can be mixed altogether.

---

Note: it is not doable to simply use the jarg module as it would install PyYAML, and we want to keep our requirements light. I also checked other modules like html-json-forms but I prefer to ship our own code for the same reason: no new module to package. At the end, the new code is maintainable enough, and quite small.

[codecov-commenter]

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[isidentical]

Merging this to feature/nested-json.