cmd/deadcode: suppress marker interface method reporting by default

[FolkLoreee]

The deadcode tool now suppresses marker interface methods
from the default output, as these are typically intentional interface
implementations rather than dead code. Users can include them in
reports using the new -marker flag.
Marker interface methods are unexported methods that: implement
a top-level interface and have no parameters, results, and function body.

Add Marker field to jsonFunction struct to indicate when a function
is a marker interface method, allowing users to filter or identify
these methods in custom format templates.

Fixes golang/go#75628

[FolkLoreee]

Note: cmd/deadcode/testdata/issue65915.txtar was modified so that deadcode does not erroneously flag the methods as marker methods.

This should still maintain the same behavior expectation as reported in golang/go#65915

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Patch Set 1:

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Patch Set 3:

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