Revisions: Use CSS outline as secondary non-color indicator for diff blocks

[himanshupathak95]

What?

Part of #77530.

Add CSS outline-based secondary indicators for the diff blocks. Adds a solid outline to added blocks.

Why?

outline was specifically proposed and endorsed as the correct pattern:

• Solid = added (permanent, present)
• Dashed = removed (impermanent, absent)
• Dotted = modified (changed, but still present)

outline is used to avoid layout interference and conflict with user-applied image styles. It renders correctly in High Contrast Mode.

Testing Instructions

1. Open a post with several revisions.
2. Enter the Revisions view and enable "Show changes".
3. Navigate revisions to find added/removed/modified blocks.
4. Verify each has a distinct outline shape (solid/dashed/dotted).
5. Verify content in the removed/modified blocks is fully readable.

Screenshots or screencast

See #77530 (comment) and #77530 (comment)

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Co-authored-by: himanshupathak95 <abcd95@git.wordpress.org>
Co-authored-by: t-hamano <wildworks@git.wordpress.org>
Co-authored-by: joedolson <joedolson@git.wordpress.org>
Co-authored-by: jasmussen <joen@git.wordpress.org>

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[t-hamano]

Thanks for the PR!

At least, all states are distinguishable in Windows High Contrast Mode. Let's wait for others' opinions.

State	Screenshot
Added content
Modified content
Deleted content

[jasmussen]

At a glance, this looks good to me. I know some other folks have followed this work a bit more closely than I have, I will defer to them if they have input.

[joedolson]

LGTM!

[t-hamano]

Let's consider backporting this to the 7.0 minor release. If there are any other opinions, we can make adjustments before backporting.