Add an appropriate label to each type of issue

.github/ISSUE_TEMPLATE/bug_report.md

@@ -1,6 +1,7 @@
 ---
 name: Bug Report
 about: Bug reports about the Solidity Compiler.
+labels: ["bug :bug:"]
 ---
 
 <!--## Prerequisites

.github/ISSUE_TEMPLATE/documentation_issue.md

@@ -1,6 +1,7 @@
 ---
 name: Documentation Issue
 about: Solidity documentation.
+labels: ["documentation :book:"]
 ---
 
 ## Page

.github/ISSUE_TEMPLATE/feature_request.md

@@ -1,6 +1,7 @@
 ---
 name: Feature Request
 about: Solidity language or infrastructure feature requests.
+labels: ["feature"]
 ---
 
 <!--## Prerequisites