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

[Athijegannathan Sundararajan]

On Tue, 5 Nov 2024 16:16:13 GMT, Christian Stein <cstein at openjdk.org> wrote:

> Please review this change that improves the launcher mode detection by reading the initial two characters from the started file for being a 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.
> 
> Tests of tier 1..3: _in progress_

LGTM

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Marked as reviewed by sundar (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/21910#pullrequestreview-2538724188