unindentMin() - Finds the minimum indentation of all of the lines that have non-space characters and removes the indentation accordingly for all indented lines.

unindent.py

def unindent(input_str):
    indent = None
    # Find all non-blank lines and their indentation
    for non_blank_line in re.finditer(r'^\n?(\s*)(?=\S)', input_str, re.MULTILINE):
        this_indent = non_blank_line.group(1)
        # Determine the smallest indent
        if indent is None or (indent != this_indent and indent.startswith(this_indent)):
            indent = this_indent
            if indent == '':
                break

    # If indent is found, remove it from all lines
    if indent is not None:
        return re.sub(rf'^\n?{indent}', '', input_str, flags=re.MULTILINE)
    return input_str

unindentMin.js

/**
 * Finds the minimum indentation of all of the lines that have non-space
 * characters and removes the indentation accordingly for all indented lines.
 * @param {string} text
 *   The string containing the lines of text that should be unindented.
 * @param {{trim: boolean, tabSize: number}=} opt_options
 *   Optional, defaults to `{trim: true, tabSize: 4}`.  The `trim` property
 *   indicates if leading lines should be removed along with trailing
 *   whitespaces.  The `tabSize` property indicates how many spaces will be
 *   used to replace all tab characters.
 * @returns {string}
 *   A new version of `text` with all of the minimally indented lines having
 *   no leading spacing and all other indented lines following suit.  If
 *   `opt_options.trim` is `true` all leading lines and trailing spaces will
 *   not exist.  All tab characters will be replaced with
 *   `opt_options.tabSize` amount of space characters.
 */
function unindentMin(text, opt_options) {
  opt_options = Object(opt_options);
  const tabSize = opt_options.tabSize ?? 4;
  const trim = opt_options.trim ?? true;
  text = text.replace(/\t/g, ' '.repeat(tabSize));
  if (!/(^|[\r\n])\S/.test(text)) {
    const rgx = /(^|[\r\n])((?:(?!\r|\n)\s)+)(?=(\S)?)/g;
    let min = Infinity;
    for (let match; match = rgx.exec(text);) {
      if (match[3]) {
        min = Math.min(min, match[2].length);
      }
    }
    text = text.replace(
      rgx,
      (_, start, spaces) => start + spaces.slice(min)
    );
  }
  return trim ? text.replace(/^(\s*[\r\n]+)+|\s+$/g, '') : text;
}

~io-config.jsonc

{
  // Name of the function responsable for transforming the input into the output.
  "transform": "unindentMin",
  // Where to find the code that will be eval'd.
  "files": ["unindentMin.js"],
  // Type of YourJS IO-App
  "type": "instant"
}

Here is the IO App:
https://www.yourjs.com/gist-io/8acb0f026014a6630b0b8ecbfeaf51f1