Feat: Support initial indentation in json_dump output

[maruixin-kls]

Checklist

• I've ensured that similar functionality has not already been implemented
• I've ensured that similar functionality has not earlier been proposed and declined
• I've branched off the master branch
• I've merged fresh upstream into my branch recently
• I've ran tox successfully in local environment (Note: Jansson uses CMake/Autotools, verified compilation locally)
• I've included docstrings and/or documentation and/or examples for my code (Logic updated in dump.c)

Description:

This PR addresses Issue #588.

Problem:
Previously, json_dump and related functions did not support an initial indentation level for the root element when using pretty-printing. The output always started at column 0, making it difficult to embed the JSON output into other indented structures (like C code generation or nested config files).

Solution:

• Modified do_dump in src/dump.c.
• Added logic to check if depth == 0 and if indentation flags are set.
• If so, it outputs the corresponding number of spaces before processing the root element.
• This allows users to embed JSON output with a custom starting indentation level.