RFR: 8340380: Improve source launcher's shebang script detection

[Christian Stein]

Please review this change that improves the launcher mode detection by reading the initial two characters from the started shebang script. It addresses the reported confusing error messages and also supports more shebang line variations. Including those line variations that omit the `--source` arguments like shown in the underlying issue description of JDK-8340380.

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Commit messages:
 - Add shebang test case without `--source`
 - 8340380: Improve source launcher's shebang script detection

Changes: https://git.openjdk.org/jdk/pull/21910/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21910&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340380
  Stats: 31 lines in 2 files changed: 30 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/21910.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21910/head:pull/21910

PR: https://git.openjdk.org/jdk/pull/21910