RFR: 8276124: Provide snippet support for properties files [v5]

[Pavel Rappo]

On Wed, 24 Nov 2021 12:31:36 GMT, Pavel Rappo <prappo at openjdk.org> wrote:

>> The initial integration (JDK-8266666) of JEP 413 did not support properties files. This commit rights that wrong.
>
> Pavel Rappo has updated the pull request incrementally with four additional commits since the last revision:
> 
>  - Use Hannes' regex
>    
>    Shortens regexes and makes them use fewer concepts (no lookahead).
>  - Implement Jon's suggestion (3)
>    
>    Cleans up.
>  - Implement Jon's suggestion (2)
>    
>    Substitutes regex for CoarseParser and its implementations.
>  - Implement Jon's suggestion (1)
>    
>    Uses named groups and positive lookahead to make both regexes uniform in respect to accessing the components of a match.

Thank you, Jon and Hannes. I believe this PR fully addresses the linked JBS issue. I would prefer to leave these discussions for later, unless any of them are showstoppers, and integrate the change ASAP:

   * Enum vs class
   * Custom pattern (getPattern as described by Jon)
   * The `prefix` attribute 
   * The `plain-text` identifier

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PR: https://git.openjdk.java.net/jdk/pull/6397