Fix heredoc

[aycabta]

The here-documents in middle of line are replaced with string literal. It's useful for IRB, but it doesn't fit for documentation.

For example,

def example
  puts [<<~TILDE, <<-HYPHEN, <<~'TILDE', <<-'HYPHEN', <<'AAA', <<"BBB", <<-`CCC`]
    a
      b
      c
    d
  TILDE
    1
      2
      3
    4
  HYPHEN
    w
      x
      y
    z
  TILDE
    1
      2
      3
    4
  HYPHEN
aaa
aa
a
AAA
bbb
bb
b
BBB
echo 'a'
echo 'b'
echo 'c'
CCC
end

MRI processes this code and outputs correctly. This is highlighted below:

<pre>
<span class="ruby-keyword">def</span> <span class="ruby-identifier">example</span>
  <span class="ruby-identifier">puts</span> [<span class="ruby-operator">&lt;&lt;</span><span class="ruby-operator">~</span><span class="ruby-constant">TILDE</span>, <span class="ruby-string">&quot;    a
      b
      c
    d
  TILDE
    1
      2
      3
    4
&quot;</span>, <span class="ruby-operator">&lt;&lt;</span><span class="ruby-operator">~</span><span class="ruby-string">&#39;TILDE&#39;</span>, <span class="ruby-string">&quot;    w
      x
      y
    z
  TILDE
    1
      2
      3
    4
&quot;</span>, <span class="ruby-string">&quot;aaa
aa
a
&quot;</span>, <span class="ruby-string">&quot;bbb
bb
b
&quot;</span>, <span class="ruby-value">&quot;echo &#39;a&#39;
echo &#39;b&#39;
echo &#39;c&#39;
&quot;</span>]
<span class="ruby-keyword">end</span>
</pre>

I think this is broken document.

This Pull Request fixes it:

<pre>
<span class="ruby-keyword">def</span> <span class="ruby-identifier">example</span>
  <span class="ruby-identifier">puts</span> [<span class="ruby-identifier">&lt;&lt;~TILDE</span>, <span class="ruby-identifier">&lt;&lt;-HYPHEN</span>, <span class="ruby-identifier">&lt;&lt;~&#39;TILDE&#39;</span>, <span class="ruby-identifier">&lt;&lt;-&#39;HYPHEN&#39;</span>, <span class="ruby-identifier">&lt;&lt;&#39;AAA&#39;</span>, <span class="ruby-identifier">&lt;&lt;&quot;BBB&quot;</span>, <span class="ruby-identifier">&lt;&lt;-`CCC`</span>]
<span class="ruby-value">    a
      b
      c
    d
</span><span class="ruby-identifier">  TILDE
</span><span class="ruby-value">    1
      2
      3
    4
</span><span class="ruby-identifier">  HYPHEN
</span><span class="ruby-value">    w
      x
      y
    z
</span><span class="ruby-identifier">  TILDE
</span><span class="ruby-value">    1
      2
      3
    4
</span><span class="ruby-identifier">  HYPHEN
</span><span class="ruby-value">aaa
aa
a
</span><span class="ruby-identifier">AAA
</span><span class="ruby-value">bbb
bb
b
</span><span class="ruby-identifier">BBB
</span><span class="ruby-value">echo &#39;a&#39;
echo &#39;b&#39;
echo &#39;c&#39;
</span><span class="ruby-identifier">CCC
</span><span class="ruby-keyword">end</span></pre>

[hsbt]

I understood to reason why we need TkHEREDOCBEG and TkHEREDOCEND.