Rename substitution-related Vale rules, add suggestion-level option

What does this MR do?

Capturing @sselhorn's thoughts from !187383 (merged):

Just to add another rule on a rule, we should try to use "because" instead of "as". Maybe you want to open another MR to add this to another Vale rule?

What I did here might sound like I overdid it a smidge, but let me narrate the steps:

• Considered where to add the as → because rule.
• Wordy.yml? Not really. Hrm. One of the substitution rules?
• Ugh, I know what SubstitutionWarning.yml is at a glance, but I can never remember if Substitution.yml is an error or a suggestion rule…
• Huh, it's an error-level rule. So that means we don't have a suggestion-level substitution rule?
• (renames ambiguous rule, creates new suggestion-level substitution rule)
• (plays some Stevie Wonder because it's already in my head anyway)

…and that's how Substitution.yml became SubstitutionError.yml, and I am proposing SubstitutionSuggestion.yml. Any test for as will return false positives, so I won't bother proposing it any higher than this.

Result count

$ vale --no-wrap --filter='.Name=="gitlab_base.SubstitutionSuggestion"' **/*.md

• 9,514 if we ignore case. (Ouch.)
• 663 if we look for As and make it case-sensitive.

I have a feeling this is about to become a case-sensitive rule 😁

Related issues

Related to Adds 'you can' to Wordy.yml (!187383 - merged) because it came up in discussion.

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