🎨 Show all custom theme setting errors for each file with line number

[troyciesco]

ref #573

• currently when a file has multiple custom theme setting errors, gscan only reports one at a time. This makes it difficult to debug a file if there are multiple errors.
• with this change, all errors for a file will be shown with a line number to make it easy to edit
• using the cli without the verbose tag still shows a comma-delimited list, with each file listed only once

before:

after (each file only has two errors, but i did test with 1 and 3):

cli output without --verbose:

Your theme has 1 error!
----

Errors
------
Important to fix, functionality may be degraded.

- Error: An unknown custom theme setting has been used.
Affected Files: index.hbs, partials/post-card.hbs, page.hbs, post.hbs

• The smaller footprint of this PR makes it easier to find and fix bugs in theme files without completely overhauling the checker

[troyciesco]

updating this test here makes sense, because before only @price.monthly was logged, but now @price.yearly is as well. File here: https://github.com/TryGhost/gscan/blob/main/test/fixtures/themes/001-deprecations/v5/invalid_all/post.hbs#L5

I don't think there are additional tests to add but I can if needed!

[troyciesco]

I prefer this output:

Affected Files: index.hbs, partials/post-card.hbs, page.hbs, post.hbs

to this:

Affected Files: index.hbs,partials/post-card.hbs,page.hbs,post.hbs

but if there was a reason for leaving out the spaces i can certainly switch it back.

I do think _.uniq is useful, because otherwise it looks like this:

Affected Files: index.hbs,index.hbs,partials/post-card.hbs,partials/post-card.hbs,page.hbs,page.hbs,post.hbs,post.hbs

Although, now I'm wondering if there's ever a reason you'd want duplicated filenames for different failure results that are output by this function - probably not? But can change if needed!