Fixes CMS header line height

[supernova-at]

Description

This PR adds a line-height rule to the CMS RootComponent CSS so that when headings are displayed they have enough space to render. The line-height value of 1.25em is consistent with other pages in the storefront application.

Related Issue

Closes PWA-714.

Acceptance

Verification Stakeholders

Specification

Verification Steps

1. Find a way to populate the content_heading field of the CMS page. I had to modify cms.js lines 41 and 42 manually locally
2. yarn watch:venia
3. Verify that the heading content appears and is displayed correctly

Screenshots / Screen Captures (if appropriate)

Checklist

• I have added tests to cover my changes, if necessary.
• I have added translations for new strings, if necessary.
• I have updated the documentation accordingly, if necessary.

[PWAStudioBot]

	Messages
📖	Associated JIRA tickets: PWA-714.
📖	DangerCI Failures related to missing labels/description/linked issues/etc will persist until the next push or next nightly build run (assuming they are fixed).
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