RFR: JDK-8266748: Move modifiers code to Signatures.java [v2]

[Hannes Wallnöfer]

> This change consolidates the code to generate type signature modifiers into `Signatures.TypeSignature`. 
> 
> Although this mostly consists of moving the code from `ClassWriterImpl` and `Utils` to `Signatures`, I also avoided the need to split the modifiers string when processing preview modifiers by returning a `List<String>` instead of a `String` in what used to be `Utils.modifiersToString` and is now `TypeSignature.getModifiers`.

Hannes Wallnöfer has updated the pull request incrementally with one additional commit since the last revision:

  JDK-8266748: Use configuration instead of writer where possible

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4142/files
  - new: https://git.openjdk.java.net/jdk/pull/4142/files/9970cd2e..b5c5a95c

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4142&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4142&range=00-01

  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4142.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4142/head:pull/4142

PR: https://git.openjdk.java.net/jdk/pull/4142