Fix auto-closing quotes for heredocs (#101)

* Fix auto-closing quotes for heredocs

Add triple double quotes (`"""`) as an auto-closing pair in the Elixir
language configuration to prevent extra quotes from being inserted when
starting a heredoc string.

* add auto-closing pairs for sigils

- add auto-closing pairs for triple double quotes with sigils
- since adding those auto closing pairs breaks single double quote
  pairs, add pairs for single double quotes with sigils as well
- the sigil list is from
  elixir-editors/emacs-elixir#454

Author: jarrodldavis

elixir-language-configuration.json

@@ -17,6 +17,39 @@
   "autoClosingPairs": [
     { "open": "'", "close": "'", "notIn": ["string", "comment"] },
     { "open": "\"", "close": "\"", "notIn": ["comment"] },
+    { "open": "~B\"", "close": "\"" },
+    { "open": "~C\"", "close": "\"" },
+    { "open": "~D\"", "close": "\"" },
+    { "open": "~E\"", "close": "\"" },
+    { "open": "~L\"", "close": "\"" },
+    { "open": "~N\"", "close": "\"" },
+    { "open": "~R\"", "close": "\"" },
+    { "open": "~S\"", "close": "\"" },
+    { "open": "~T\"", "close": "\"" },
+    { "open": "~U\"", "close": "\"" },
+    { "open": "~b\"", "close": "\"" },
+    { "open": "~c\"", "close": "\"" },
+    { "open": "~e\"", "close": "\"" },
+    { "open": "~r\"", "close": "\"" },
+    { "open": "~s\"", "close": "\"" },
+    { "open": "~w\"", "close": "\"" },
+    { "open": "\"\"\"", "close": "\"\"\"" },
+    { "open": "~B\"\"\"", "close": "\"\"\"" },
+    { "open": "~C\"\"\"", "close": "\"\"\"" },
+    { "open": "~D\"\"\"", "close": "\"\"\"" },
+    { "open": "~E\"\"\"", "close": "\"\"\"" },
+    { "open": "~L\"\"\"", "close": "\"\"\"" },
+    { "open": "~N\"\"\"", "close": "\"\"\"" },
+    { "open": "~R\"\"\"", "close": "\"\"\"" },
+    { "open": "~S\"\"\"", "close": "\"\"\"" },
+    { "open": "~T\"\"\"", "close": "\"\"\"" },
+    { "open": "~U\"\"\"", "close": "\"\"\"" },
+    { "open": "~b\"\"\"", "close": "\"\"\"" },
+    { "open": "~c\"\"\"", "close": "\"\"\"" },
+    { "open": "~e\"\"\"", "close": "\"\"\"" },
+    { "open": "~r\"\"\"", "close": "\"\"\"" },
+    { "open": "~s\"\"\"", "close": "\"\"\"" },
+    { "open": "~w\"\"\"", "close": "\"\"\"" },
     { "open": "`", "close": "`", "notIn": ["string", "comment"] },
     { "open": "(", "close": ")" },
     { "open": "{", "close": "}" },