This gets the next key and value from an object. This function is better
because it will skip key/value pairs that don't fit in the array to hold
the key.
The previous code patter would stop parsing the object as soon as a key
larger than the available space was found.
Add struct spa_error_location that holds information about some parsing
context such as the line and column number, error and line fragment
with the error.
Make spa_json_get_error() fill in the spa_error_location instead. Add
some error codes to the error state and use this to add a parsing reason
to the location.
Add a debug function to log the error location in a nice way. Also
add a FILE based debug context to log to any FILE.
Replace pw_properties_check_string() with
pw_properties_update_string_checked() and add
pw_properties_new_string_checked(). The check string behaviour can still
be done by setting props to NULL but the main purpose is to be able to
avoid parsing the json file twice in the future.
When using the old pw_properties_update_string(), log a warning to the
log when we fail to parse the complete string.
Use the new checked functions and the debug functions to report about
parsing errors in the tools and conf parsing.
This gives errors like:
```
> pw-loopback --playback-props '{ foo = [ f : g ] }'
error: syntax error in --playback-props: Invalid array separator
line: 1 | { foo = [ f : g ] }
col: 14 | ^
```
Support reading from stdin and report syntax errors.
Also don't do extra spa_json_enter when inserting a dummy "{" when
parsing files with top-level keys. In this case the tokenizer is
already "entered" after spa_json_init, and will give parse error when we
are not inserting the closing "}" for the dummy "{".
