Add go:embed directive support for interpreted packages

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What this task asks for

Support //go:embed directives so interpreted package variables can receive embedded file contents and embed.FS values. Support `//go:embed` directives that embed file contents into package-level variables. The variable must hold its embedded content by the time the first interpreted statement executes. The interpreter's standard variable initialization must not overwrite it. `embed.FS` -- one or more files as a read-only filesystem. For `string` and `[]byte`, patterns must resolve to exactly one file. Opened directories implement `fs.ReadDirFile`.