Add issue templates

Author: ladisgin

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Create a report to help us improve
+title: "[BUG]"
+labels: bug
+assignees: ''
+
+---
+
+**Description**
+There are hundreds of error messages in the UTBot log. Looks like the problem is in the concrete executor.
+
+**To Reproduce**
+Steps to reproduce the behavior:
+1. Open the 'X' project in IDE
+2. Use plugin to generate tests
+3. Run the generated test
+
+**Expected behavior**
+Tests are supposed to be generated.
+
+**Actual behavior**
+An error test is generated with information about errors in the concrete executor.
+
+**Visual proofs (screenshots, logs)**
+```cpp
+void testFail_errors() {
+// Couldn't generate some tests. List of errors:
+}
+```
+
+**Environment**
+_Substitute this text with an information that can help us to recreate the environment. For instance, specify server and client operation systems, UTBotCpp version and so on._
+
+**Additional context**
+Add any other context about the problem here.

.github/ISSUE_TEMPLATE/feature_request.md

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+---
+name: Feature request
+about: Suggest an idea for this project
+title: ''
+labels: enhancement
+assignees: ''
+
+---
+
+**Description**
+Substitute this text with a concise description of the problem. Please be clear what parts of the problem are considered to be in-scope and out-of-scope.
+
+**Expected behavior**
+Substitute the text with a clear and concise description of what you want to happen.
+
+**Environment**
+Substitute the text with steps to set up the environment for testing of the expected behavior.
+
+**Potential solution**
+Substitute this text with a potential solution.
+
+**Context**
+Substitute this text with information that can be crucial for understanding the problem.