Better error message for mixed tables

[Chris Chang]

"mixed tables are not allowed" doesn't actually say why it's bad. It's also easy to misinterpret this since mixed tables are valid, but are just heavily discouraged.

error[mixed_table]: mixed tables are not allowed
  ┌─ mixed_table.lua:2:5
  │  
2 │ ╭     "",
3 │ │     a = b,
  │ ╰─────────^
  │  
  = help: change this table to either an array or dictionary