Apply margin to all pre in .nicegui-markdown

[evnchn]

Motivation

Fix #5264, where the margin is inconsistent when language=''

Implementation

• Changed the selector from .nicegui-markdown .codehilite pre to .nicegui-markdown pre and updated the margin to 0.5rem 0.5rem to provide both horizontal and vertical margin to both codehilite and non-codehilite code blocks.
• Removed unnecessary padding from .nicegui-code .codehilite.

Visual difference

Before:

After:

Progress

• I chose a meaningful title that completes the sentence: "If applied, this PR will..."
• The implementation is complete.
• Pytests have been added (or are not necessary).
• Documentation has been added (or is not necessary).