fix: align button height with Monaco metrics

[dankeboy36]

• aligns the button's height with Monaco by using line-height: 18px; (same as VS Code's .monaco-text-button, see https://github.com/microsoft/vscode/blob/91b31240fb8a0afcbff105d69cd7fee7af9dd23f/src/vs/base/browser/ui/button/button.css#L17).
• mirrors Monaco's 4px vertical padding so the box model sums to ~27.8px, while letting button groups add their 1px divider offset via --vsc-base-additional-right-padding (falls back to 0 elsewhere).
• Keep icon-only and empty buttons at the same height by retaining Monaco's 4px top/bottom padding and a 26px min-width for glyph-only controls (see https://github.com/microsoft/vscode/blob/91b31240fb8a0afcbff105d69cd7fee7af9dd23f/src/vs/base/browser/ui/button/button.css#L10).

Closes: vscode-elements/elements#554

Before:

After: