Regulate top-level arrays

[isidentical]

Starting with the initial implementation (and later with the second one), we had a special/hidden feature called 'top-level arrays':

$ http --offline --print=B pie.dev/post []:=1 []:=2 []:=3

[
    1,
    2,
    3
]

This is a powerful syntax, since there weren't a way of creating/sending arrays in the past (without sending them as raw data). One thing that we skipped between two different implementations was to make this more stricter.

@ducaale previously started a discussion but we did not made a resolution back then due to worries over this would block the empty-key usage. Now that those are answered in #1286, I think we should be able to make the handling of top-level lists more seriously.

New handling

• This patch forbids the usage of key-paths without a preceding key [foo]=bar (and variants of this)
• If you need to embed such a structure, you can still continue to use :={"foo": "bar"}.
• Mixing top-level list usage with other keys is now forbidden. E.g:

$ http --offline --print=B pie.dev/post a:=1 []:=2
{
    "": [
        2
    ],
    "a": 1
}

[codecov-commenter]

Codecov Report

Merging #1292 (5d39576) into master (4d7d6b6) will decrease coverage by 0.76%.
The diff coverage is 95.08%.

@@            Coverage Diff             @@
##           master    #1292      +/-   ##
==========================================
- Coverage   97.28%   96.52%   -0.77%     
==========================================
  Files          67       85      +18     
  Lines        4235     6012    +1777     
==========================================
+ Hits         4120     5803    +1683     
- Misses        115      209      +94     

Impacted Files	Coverage Δ
tests/test_binary.py	100.00% <ø> (ø)
httpie/compat.py	31.11% <27.90%> (-68.89%)	⬇️
tests/conftest.py	77.14% <58.33%> (-9.82%)	⬇️
httpie/output/lexers/http.py	59.09% <59.09%> (ø)
tests/test_ssl.py	91.01% <66.66%> (-3.93%)	⬇️
httpie/manager/__main__.py	82.35% <82.35%> (ø)
httpie/output/lexers/metadata.py	82.35% <82.35%> (ø)
httpie/models.py	94.23% <90.00%> (-3.14%)	⬇️
tests/test_uploads.py	97.70% <92.59%> (-2.30%)	⬇️
httpie/manager/core.py	92.85% <92.85%> (ø)
... and 71 more

---

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