[foreign-preview] RFR: 8281412: MemorySegment::map should live in FileChannel [v4]

[Julia Boes]

> This change moves the factory for mapped memory segments to `FileChannel`, as a new instance method `map`, which moves it closer to the existing `FileChannel::map` that returns a `MappedByteBuffer`.  
> 
> Tests are adjusted accordingly and a new test for custom file channels (that do/don't override the method) is added.

Julia Boes has updated the pull request incrementally with one additional commit since the last revision:

  Address PR comment: update java doc

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Changes:
  - all: https://git.openjdk.java.net/panama-foreign/pull/668/files
  - new: https://git.openjdk.java.net/panama-foreign/pull/668/files/16895b36..737add2c

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=668&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=panama-foreign&pr=668&range=02-03

  Stats: 25 lines in 1 file changed: 9 ins; 7 del; 9 mod
  Patch: https://git.openjdk.java.net/panama-foreign/pull/668.diff
  Fetch: git fetch https://git.openjdk.java.net/panama-foreign pull/668/head:pull/668

PR: https://git.openjdk.java.net/panama-foreign/pull/668