feat: Add .list.json_encode()

[moritzwilksch]

fixes #14029

adds json_encode() to the .list. namespace to serialize lists to JSON strings.

There are previous discussion about adding this to the root name space but I can't tell how much more work that would be and I'm not rustacean.

ToDo

• rename to json_encode for consistency
• docs
• Series.list namespace?
• test w/ complex types (objects, structs, nested lists)
• test round trip encode decode
• consolidate with feat: JSON encoding in the root expression namespace pl.Expr.json_encode #18396

[moritzwilksch]

@orlp you suggested moving it to the root namespace. I could also try to revive #18396 instead, seems like it was pretty close to done or am I missing something there?