Support nested curly braces in placeholders

[dmmqz]

Currently nested braces in placeholders are not supported (for example: {{path/to/{a,b}/{x,y,z}/{h,i,j}}}). This has also been discussed in #17189. As a result, the tldr bot will raise a warning in translated pages with such placeholders.

[sebastiaanspeck]

Maybe #17189 (comment) could be a solution for this