fix(chart): make xticks count tick positions

[lovasoa]

Summary

• translate numeric xticks values from requested tick positions to ApexCharts intervals
• assert the numeric chart fixture renders exactly the requested number of x-axis labels
• update the chart documentation and unreleased changelog entry

Tests

• npm run format
• npm test
• SQLPAGE_FIXTURE_BASE=http://127.0.0.1:18081 SQLPAGE_BINARY=/home/ophir/dev/SQLPage/target/debug/sqlpage npx playwright test fixtures/chart/test.ts --project=fixtures --reporter=line