gh-113149: improve error when json has trailing comma

[carsonRadtke]

Fixes: gh-113149

Adds two new JSONDecodeError types: "Illegal trailing comma before end of {array,object}".

Some test's expected error location had to change because the error now exists with the existence of the comma, rather than the non-existence of a following object.

Had to introduce a Py_ssize_t comma_idx; because we want to ensure that

{ "a": 123   ,            } 
             ^ <- error reported here

Behavior after the PR:

$ echo '[,' | ./python.exe -m json.tool
Expecting value: line 1 column 2 (char 1)
$ echo '{,}' | ./python.exe -m json.tool
Expecting property name enclosed in double quotes: line 1 column 2 (char 1)
$ echo '[123,]' | ./python.exe -m json.tool
Illegal trailing comma before end of array: line 1 column 5 (char 4)
$ echo '{"a":123,}' | ./python.exe -m json.tool
Illegal trailing comma before end of object: line 1 column 9 (char 8)
$ echo '[123,  ]' | ./python.exe -m json.tool
Illegal trailing comma before end of array: line 1 column 5 (char 4)
$ echo '{"a":123,  }' | ./python.exe -m json.tool
Illegal trailing comma before end of object: line 1 column 9 (char 8)
$ echo '{"spam":42,}' | ./python.exe -m json.tool
Illegal trailing comma before end of object: line 1 column 11 (char 10)

• Issue: Improving error message with trailing comma in json #113149

[serhiy-storchaka]

Very good. Please fix style add add test cases with spaces between comma and closing brace.

[serhiy-storchaka]

LGTM.

[serhiy-storchaka]

Thank you for your contribution @carsonRadtke.