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

[Pavel Rappo]

> 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.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6397/files
  - new: https://git.openjdk.java.net/jdk/pull/6397/files/19e8db24..d22cbf7d

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6397&range=04
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6397&range=03-04

  Stats: 53 lines in 1 file changed: 3 ins; 37 del; 13 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6397.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6397/head:pull/6397

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