fix: tags when using trailing + regular comments

[lrstanley]

This PR fixes an issue with the new trailing comment functionality that I didn't take into consideration when first implementing it.

When you have the following configuration:

message URL {
  // scheme defines what protocol schema should be used (e.g. http, https, etc).
  string scheme = 1; // @inject_tag: valid:"http|https"
  [...]
}

This would cause no tags to be added, because if any comment (even one without tags) was defined in the Doc struct field, it wouldn't try and parse the trailing comments.

This PR does two things:

• Fixes the above issue, so you can still have a regular comment in the Doc field, and have trailing tags defined.
• If tags are defined in both locations, the trailing comment takes precedence.

[coveralls]

Pull Request Test Coverage Report for Build 77

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 71.505%

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Totals
Change from base Build 74:	0.0%
Covered Lines:	133
Relevant Lines:	186

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💛 - Coveralls

[lrstanley]

@favadi are you ok with me merging changes in myself as long as they're not large, or would you prefer to review?

[favadi]

@lrstanley feel free to merge anything yourself, as long as it is backward compatible.