feat: add Table SetSeparator config

[cassdeckard]

Summary

Add configuration for Table's SetSeparator so the character between cells can be set via YAML (e.g. space, vertical bar, or tview's Borders.Vertical).

Background

Discovered while planning a tview presentation demo implementation via tviewyaml. The Table slide demonstrates different separator styles: ' ', Borders.Vertical, and bordered mode.

Demo Usage

Demo uses SetSeparator(' '), SetSeparator(Borders.Vertical); not in config/types.go.

Proposed Approach

• Add separator to Table primitive config: string for single char (e.g. " "), or named values like "vertical", "horizontal", "none"
• Map to rune or tview.Borders.* in builder
• In builder/builder.go populateTableData, call SetSeparator before/after setting cells

Relevant Files

• config/types.go – add separator to Primitive (Table)
• builder/builder.go – populateTableData
• docs/tview-coverage.md – document once implemented