[outtmpl] Ensure ASCII in json and add option for Unicode

Closes #5236

README.md

@@ -1189,7 +1189,7 @@ The field names themselves (the part inside the parenthesis) can also have some
 
 1. **Default**: A literal default value can be specified for when the field is empty using a `|` separator. This overrides `--output-na-placeholder`. E.g. `%(uploader|Unknown)s`
 
-1. **More Conversions**: In addition to the normal format types `diouxXeEfFgGcrs`, yt-dlp additionally supports converting to `B` = **B**ytes, `j` = **j**son (flag `#` for pretty-printing), `h` = HTML escaping, `l` = a comma separated **l**ist (flag `#` for `\n` newline-separated), `q` = a string **q**uoted for the terminal (flag `#` to split a list into different arguments), `D` = add **D**ecimal suffixes (e.g. 10M) (flag `#` to use 1024 as factor), and `S` = **S**anitize as filename (flag `#` for restricted)
+1. **More Conversions**: In addition to the normal format types `diouxXeEfFgGcrs`, yt-dlp additionally supports converting to `B` = **B**ytes, `j` = **j**son (flag `#` for pretty-printing, `+` for Unicode), `h` = HTML escaping, `l` = a comma separated **l**ist (flag `#` for `\n` newline-separated), `q` = a string **q**uoted for the terminal (flag `#` to split a list into different arguments), `D` = add **D**ecimal suffixes (e.g. 10M) (flag `#` to use 1024 as factor), and `S` = **S**anitize as filename (flag `#` for restricted)
 
 1. **Unicode normalization**: The format type `U` can be used for NFC [unicode normalization](https://docs.python.org/3/library/unicodedata.html#unicodedata.normalize). The alternate form flag (`#`) changes the normalization to NFD and the conversion flag `+` can be used for NFKC/NFKD compatibility equivalence normalization. E.g. `%(title)+.100U` is NFKC
 

yt_dlp/YoutubeDL.py

@@ -1249,7 +1249,7 @@ class YoutubeDL:
             elif fmt[-1] == 'j':  # json
                 value, fmt = json.dumps(
                     value, default=_dumpjson_default,
-                    indent=4 if '#' in flags else None, ensure_ascii=False), str_fmt
+                    indent=4 if '#' in flags else None, ensure_ascii='+' not in flags), str_fmt
             elif fmt[-1] == 'h':  # html
                 value, fmt = escapeHTML(str(value)), str_fmt
             elif fmt[-1] == 'q':  # quoted