Ruby: Add heredoc literals

Adds support for basic heredoc-style string literals in the Ruby syntax.

https://ruby-doc.org/core-2.7.0/doc/syntax/literals_rdoc.html#label-Here+Documents+-28heredocs-29

There are several more styles of heredocs to add as well.

components/prism-ruby.js

@@ -102,6 +102,21 @@
 			inside: {
 				'interpolation': interpolation
 			}
+		},
+		{
+			pattern: /<<([a-z_]\w*)[\r\n](?:.*[\r\n])*?\1/i,
+			alias: 'heredoc-string',
+			greedy: true,
+			inside: {
+				'delimiter': {
+					pattern: /^<<[a-z_]\w*|[a-z_]\w*$/i,
+					alias: 'symbol',
+					inside: {
+						'punctuation': /^<<([-~]?)/
+					}
+				},
+				'interpolation': interpolation
+			}
 		}
 	];
 

examples/prism-ruby.html

@@ -7,7 +7,10 @@ <h2>Comments</h2>
 
 <h2>Strings</h2>
 <pre><code>"foo \"bar\" baz"
-'foo \'bar\' baz'</code></pre>
+'foo \'bar\' baz'
+&lt;&lt;STRING
+  here's some #{string} contents
+STRING</code></pre>
 
 <h2>Regular expressions</h2>
 <pre><code>/foo?[ ]*bar/</code></pre>

tests/languages/ruby/string_feature.test

@@ -36,6 +36,10 @@ bar"
 %x[foo #{ 42 }]
 %x<foo #{ 42 }>
 
+<<STRING
+  foo #{42} bar
+STRING
+
 ----------------------------------------------------
 
 [
@@ -279,9 +283,23 @@ bar"
             ["delimiter", "}"]
         ]],
         ">"
+    ]],
+    ["string", [
+      ["delimiter", [
+        ["punctuation", "<<"], 
+        "STRING"
+      ]],
+      "\r\n  foo ",
+      ["interpolation", [
+        ["delimiter", "#{"],
+        ["number", "42"],
+        ["delimiter", "}"]
+      ]],
+      " bar\r\n",
+      ["delimiter", ["STRING"]]
     ]]
 ]
 
 ----------------------------------------------------
 
-Checks for strings and string interpolation.
+Checks for strings and string interpolation.