chore: add GitHub issue templates and PR template

[kirich1409]

Context

The repository has no .github/ISSUE_TEMPLATE/ or .github/pull_request_template.md. Without templates, bug reports and feature requests arrive in freeform, making triage harder and AI-agent-assisted processing less reliable.

Acceptance criteria

Issue templates (.github/ISSUE_TEMPLATE/)

• bug_report.yml — structured form: description, steps to reproduce, expected vs actual, library version, platform (Android / iOS / JVM)
• feature_request.yml — motivation, proposed API, alternatives considered
• config.yml — disable blank issues, add link to Discussions for questions

PR template (.github/pull_request_template.md)

• Linked issue (Closes #)
• Type of change (bug fix / new feature / breaking change / docs)
• Checklist: tests added, docs updated, spotlessCheck passes, CHANGELOG updated
• Notes for reviewers (optional section)

Notes

Use YAML-based issue forms (not legacy Markdown templates) — they render as structured forms in the GitHub UI and are easier for automation to parse.