---input---
f"{f'{inner}'}"

---tokens---
'f'           Literal.String.Affix
'"'           Literal.String.Double
'{'           Literal.String.Interpol
'f'           Literal.String.Affix
"'"           Literal.String.Single
'{'           Literal.String.Interpol
'inner'       Name
'}'           Literal.String.Interpol
"'"           Literal.String.Single
'}'           Literal.String.Interpol
'"'           Literal.String.Double
'\n'          Text.Whitespace