Unterminated single quotes may run arbitrary commands

bug: The shell does not create and error upon a wrong let binding with single quotes. It appears to run arbitrary commands, when further single quotes appear. If you are lucky, it creates a core dump.

expect: Not silently accepting a wrong grammar (for files). The REPL should also indicate, that the string is not terminated.

related: #959 (closed) #949 (closed)

code: input

let a = 'ab''

expect: output

ion: syntax error: unterminated single quote

version: f05b74e7

interaction: Alacritty with ion

context: terminal multiplexers may prevent faster core dump