feat(highcharts): implement indicator-rsi (#3261)

## Implementation: `indicator-rsi` - highcharts

Implements the **highcharts** version of `indicator-rsi`.

**File:** `plots/indicator-rsi/implementations/highcharts.py`

**Parent Issue:** #3229

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:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20795216038)*

Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

Author: github-actions[bot]

plots/indicator-rsi/metadata/highcharts.yaml

@@ -0,0 +1,19 @@
+# Per-library metadata for highcharts implementation of indicator-rsi
+# Auto-generated by impl-generate.yml
+
+library: highcharts
+specification_id: indicator-rsi
+created: '2026-01-07T20:21:02Z'
+updated: '2026-01-07T20:21:02Z'
+generated_by: claude-opus-4-5-20251101
+workflow_run: 20795216038
+issue: 3229
+python_version: 3.13.11
+library_version: unknown
+preview_url: https://storage.googleapis.com/pyplots-images/plots/indicator-rsi/highcharts/plot.png
+preview_thumb: https://storage.googleapis.com/pyplots-images/plots/indicator-rsi/highcharts/plot_thumb.png
+preview_html: https://storage.googleapis.com/pyplots-images/plots/indicator-rsi/highcharts/plot.html
+quality_score: null
+review:
+  strengths: []
+  weaknesses: []