Fix dropdown button margin to prevent overlap

[DustRespirator]

Problem

In the terminal panel, the "Launch Profile..." dropdown (chevron-down icon) partially overlaps the adjacent "+" button. This overlap is caused by the global CSS rule in dropdown.css:

.monaco-dropdown-with-primary > .dropdown-action-container > .monaco-dropdown > .dropdown-label .codicon[class*='codicon-'] {
	margin-left: -3px;
}

Also, in Copilot panel, the "More..." button next to the "Send and Dispatch" button uses the same CSS rule and has the same overlap.

This negative margin was introduced in cc66860 (2018) when copying styles from the Monaco editor, and later slightly adjusted in b65fc85 with the intent to improve spacing. However, the overlap in specific UI elements still exists.

Fix

This PR removes or adjusts the margin-left value only for the dropdown button, preventing visual overlap with the adjacent action button and any other button appear on the left side of dropdown button, without affecting dropdowns in other parts of the workbench.

Visual comparison

Before (current stable):

The dropdown chevron overlaps the "+" button on the right. (Or vice versa, depends on which button is highlighted)

After (this PR):

Chevron is correctly spaced; both buttons are visually distinct and aligned.

Impact

This is a purely visual/UI fix with no impact on behavior or functionality. No tests were added. It resolves a minor but long-standing visual regression in the UI.