Add issue template for extension request

Closes #1696

.github/ISSUE_TEMPLATE/bug-report.md

+---
+name: Bug report
+about: Report a bug and help us improve
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+<!--
+Please see https://github.com/cdr/code-server/blob/master/doc/FAQ.md#how-do-i-debug-issues-with-code-server
+and include any logging information relevant to the issue.
+
+Please search for existing issues before filing.
+
+If you can reproduce the issue on vanilla VS Code,
+please file the issue at the VS Code repository instead.
+
+Provide screenshots if applicable.
+
+Please fill in the issue template and try to be as detailed
+and clear as possible!
+-->
+
+- Web Browser:
+- Local OS:
+- Remote OS:
+- Remote Architecture:
+- `code-server --version`:

.github/ISSUE_TEMPLATE/extension-request.md

+---
+name: Extension request
+about: Request an extension missing from the code-server marketplace
+title: ''
+labels: extension-request
+assignees: cmoog
+
+---
+
+<!--
+Details on the code-server extension marketplace are at
+
+https://github.com/cdr/code-server/blob/master/doc/FAQ.md#whats-the-deal-with-extensions
+
+Please fill in the issue template!
+-->
+
+- [ ] Extension name:
+- [ ] Extension GitHub or homepage:

.github/ISSUE_TEMPLATE/feature-request.md

+---
+name: Feature request
+about: Suggest an idea
+title: ''
+labels: feature
+assignees: ''
+
+---
+
+<!--
+Please search for existing issues before filing.
+
+Please describe the feature as clearly as possible!
+-->

.github/issue_template.md

 <!--
 Please file all questions and support requests at https://www.reddit.com/r/codeserver/
 The issue tracker is only for bugs.
-
-Please see https://github.com/cdr/code-server/blob/master/doc/FAQ.md#how-do-i-debug-issues-with-code-server
-and include any logging information relevant to the issue.
-
-Please search for existing issues before filing.
-
-If you can reproduce the issue on vanilla VS Code,
-please file the issue at the VS Code repository instead.
-
-Please fill in the issue template and try to be as detailed
-and clear as possible!
 -->
-
-- Web Browser:
-- Local OS:
-- Remote OS:
-- Remote Architecture:
-- `code-server --version`: