Fix code terminated check with heredoc and backtick

[tompng]

Fixes #361 and also fixes similar problem with def `();end self.`('sleep 1') (#334)

Heredoc

This code shows a sample of finding heredoc_beg corresponding to heredoc_end.

<<A+<<B+%[#{<<C+<<D
#{<<E+<<F
#{<<G+<<H
G
H
}
E
F
}
A
B
C
D
}]

In line 1, there are heredocs(A,B,C,D). These heredocs will start from the next line. (pushed into pending_heredoc)
Heredoc will be closed in an order A → B → C → D, so it should be reversed and pushed to start_token when reached a new line.

Backtick

irb(main):001:0` def `();end
irb(main):002:0` 
irb(main):003:0` self.`('sleep 1')
irb(main):004:0` 
# expect
irb(main):001:0> def `(); end
=> :`
irb(main):002:0> self.`('sleep 1')
=> ""

Checks if backtick is `command` or not.

`sleep 1`         #<Ripper::Lexer::Elem: on_backtick@1:0 BEG token: "`">
self.`('sleep 1') #<Ripper::Lexer::Elem: on_backtick@1:5 ARG token: "`">
def `(arg); end   #<Ripper::Lexer::Elem: on_backtick@1:4 ENDFN token: "`">

lexer.parse.sort_by(&:pos)

Test that I added failed in ruby 2.5.0 and 2.6.0 without the third commit.
Result of Ripper::Lexer.new("<<A\nA\n").parse (used in < 2.7.0) is not ordered by position.

[#<struct Ripper::Lexer::Elem pos=[1, 0], event=:on_heredoc_beg, tok="<<A", state=EXPR_BEG>,
 #<struct Ripper::Lexer::Elem pos=[2, 0], event=:on_heredoc_end, tok="A\n", state=EXPR_BEG>,
 #<struct Ripper::Lexer::Elem pos=[1, 3], event=:on_nl, tok="\n", state=EXPR_BEG>]

[st0012]

The tests seem to pass without pending_heredocs.first ||. Would you mind adding another test to cover this scenario?

[tompng]

I added these test cases.

['<<A;<<B', "<<A;<<B\n", "%W[\#{<<A;<<B", "%W[\#{<<A;<<B\n"]`

process_string_literal of these code should all be <<A.
without pending_heredocs.first ||, it will be

[nil, '<<A', '%W[', '<<A']

[st0012]

👍 thx

[st0012]

@k0kubun would you mind giving it a look too? thx

[k0kubun]

Looks good to me. Thank you!