Update issue templates

Author: lenaorobei

.github/ISSUE_TEMPLATE/bug_report.md

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+---
+name: Bug report
+about: Technical issue with Magento Coding Standard
+title: ''
+labels: bug
+assignees: ''
+
+---
+
+### Preconditions
+<!---
+Provide any important information on the environment where bug is reproducible.
+-->
+1.
+2.
+
+### Steps to reproduce
+<!---
+Important: Provide a set of clear steps to reproduce this bug. We can not provide support without clear instructions on how to reproduce.
+-->
+1.
+2.
+
+### Expected result
+<!--- Tell us what do you expect to happen. -->
+1.
+2.
+
+### Actual result
+<!--- Tell us what happened instead. Include error messages and issues. -->
+1.
+2.

.github/ISSUE_TEMPLATE/existing-rule-enhancement.md

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+---
+name: Existing rule enhancement
+about: Improvement for existing Magento Coding Standard rule
+title: ''
+labels: enhancement
+assignees: ''
+
+---
+
+### Description
+<!--- Describe the enhancement you would like to make. -->
+
+### Expected behavior
+<!--- What is the expected behavior of this improvement? -->
+
+### Benefits
+<!--- How do you think this feature would improve Magento Coding Standard? -->
+
+### Additional information
+<!--- What other information can you provide about the desired feature? -->

.github/ISSUE_TEMPLATE/existing-sniffs-enhancement.md

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+---
+name: Existing sniffs enhancement
+about: Improvement for existing Magento Coding Standard rule
+title: ''
+labels: enhancement
+assignees: ''
+
+---
+
+### Description
+<!--- Describe the enhancement you would like to make. -->
+
+### Expected behavior
+<!--- What is the expected behavior of this improvement? -->
+
+### Benefits
+<!--- How do you think this feature would improve Magento Coding Standard? -->
+
+### Additional information
+<!--- What other information can you provide about the desired feature? -->

.github/ISSUE_TEMPLATE/new-rule-proposal.md

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+---
+name: New rule proposal
+about: Proposal for new Magento Coding Standard rule
+title: ''
+labels: proposal
+assignees: ''
+
+---
+
+### Rule
+<!--- Provide a high level explanation of the rule -->
+
+### Reason
+<!--- Explain, why this rule improves code quality -->
+
+### Implementation
+<!--- If possible: how to technically check the compliance to this rule -->