Add <main> to "Offset post without featured image"

[carolinan]

Description
Adds the <main> HTML tag to the pattern. All templates must include a main, both because all content needs to be inside a landmark, but also because in block themes, the automated skip link requires it.

Indent the blocks in the file to improve the readability of the code.

Partial for #278

Screenshots

Testing Instructions

Apply the PR and go to Appearance > Editor > Templates.
Select the single post template.
In the settings sidebar, open the Template tab.
Open the Design panel and select the "Offset post without featured image" design.
Open the list view.
Select the group block that is between the header and footer.
In the block settings sidebar, open the Advanced panel.
Confirm that main is selected in the HTML element option.
Save.
View a single post on the front.
Confirm that there are no issues with the layout, the template should look identical before and after adding the main.

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Co-authored-by: carolinan <[email protected]>
Co-authored-by: juanfra <[email protected]>

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[juanfra]

Looks good - I found a couple of is-style-default appearances that we may want to remove now.

[carolinan]

It looks like there were many changes to the file, but that is mostly because of the indentation.

I removed is-style-default.
I renamed it to template-single-offset based on the discussion here: #244
And I replaced the comments area markup with the comments pattern, to avoid repeating code.
I checked that the previous comment area was identical to the pattern.

To do:

• Internationalization - add translation functions
• Check if the Post navigation pattern can be used, to avoid repeating code.

[juanfra]

Looks good to me, thanks Carolina!

For i18n, I've added this PR that will probably help us with that.

Regarding reusing the post pagination pattern, I believe we could tackle that in a separate template, and do it in all the templates where it's needed.

[carolinan]

I found that the spacing is different between the post navigation in this pattern and in the the post navigation pattern.
It can be sorted, but yeah lets get these fixes in first.