feat(issues): add severity filtering support to list command

[faisal-990]

Description:
#247 Filter issues by severity

This PR adds a --severity (shorthand -s) flag to the issues list command, allowing users to filter issues by severity directly from the CLI. This improves issue triage by enabling developers to quickly surface high-priority findings without additional post-processing.

Key Changes:

• Added SeverityArg to IssuesListOptions and wired it to the issues list command with shorthand -s.
• Implemented filterIssuesBySeverity in utils.go.
  • Supports multiple severity flags (e.g. --severity critical --severity major) using OR semantics.

Validation & Normalization:

• Validates user input against supported DeepSource severities: CRITICAL, MAJOR, MINOR.
• Normalizes both user input and API values to uppercase to ensure case-insensitive matching.

Testing:

• Added unit tests for severity filtering in list_test.go, covering:
  • Single and multiple severity filters
  • Invalid severity input handling
  • Duplicate severity flags
• Introduced isolated mock data at testdata/dummy/issues_severity.json to avoid coupling with existing fixtures.

[jai-deepsource]

Will be merged as part of #264