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

[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`.

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Commit messages:
 - JDK-8266748: Make getModifiers() non-static
 - JDK-8266748: Move modifiers code to Signatures.java

Changes: https://git.openjdk.java.net/jdk/pull/4142/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4142&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266748
  Stats: 261 lines in 5 files changed: 118 ins; 123 del; 20 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