Make `literal_type` indicator detect more open chunks of code, not only strings

Description

Literal type (or @ltype) is this character in the end of prompt:

          ↓
2.6.3 :001 > 'closed string'
2.6.3 :002"> %Q[not closed array of words
2.6.3 :003/> /not closed regexp
2.6.3 :004`> `echo open backtick
          ↑

Documentation says that it detects string types:

#     %l    # type of string(", ', /, ]), `]' is inner %w[...]

But it already does more than that, detecting regular expressions and some array types:

2.6.3 :003/> /not closed regexp
# Array of words
2.6.3 :003]> %w[word word

So I think it would be convenient if it detected open parentheses, square and curly braces as well. For example:

2.6.3 :003)> parentheses = (a + b * c
2.6.3 :003]> array = [1, 'milk'

Start by locating the literal_type/@ltype implementation and any tests covering its current string, regexp, and array detection. Extend the behavior to recognize open parentheses, square brackets, and curly braces, then verify that the prompt indicator reports each example as open.