Implement JFR string encoding type 2 (UTF-16)

While investigating a corrupt JFR file, we found that string encoding type 2 is not implemented in the parser. The current code in `parser/parser.go` has a `//todo implement 2` comment for this case.

When a `jdk.FileWrite` event contains a string with encoding type 2, parsing fails with `unknown string type 2`. This encoding type appears to be UTF-16 (used by the JVM for some string values).

Relevant code: `parser/parser.go` in the `string()` method.

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co-reviewed by opus

Start in parser/parser.go, specifically the string() method and its `//todo implement 2` case. Reproduce parsing of a jdk.FileWrite event containing a type 2 string, then trace the existing string decoding paths. Done means type 2 UTF-16 strings parse successfully without the `unknown string type 2` error.

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