repl: Simplify paren wrap, continuation-detection

This simplifies the logic that was in isSyntaxError, as well as the
choice to wrap command input in parens to coerce to an expression
statement.

1. Rather than a growing blacklist of allowed-to-throw syntax errors,
just sniff for the one we really care about ("Unexpected end of input")
and let all the others pass through.

2. Wrapping {a:1} in parens makes sense, because blocks and line labels
are silly and confusing and should not be in JavaScript at all.
However, wrapping functions and other types of programs in parens is
weird and required yet *more* hacking to work around.  By only wrapping
statements that start with { and end with }, we can handle the confusing
use-case, without having to then do extra work for functions and other
cases.

This also fixes the repl wart where `console.log)(` works in the repl,
but only by virtue of the fact that it's wrapped in parens first, as
well as potential side effects of double-running the commands, such as:

    > x = 1
    1
    > eval('x++; throw new SyntaxError("e")')
    ... ^C
    > x
    3