tcl: Return scratchpad result.json and result.string as tcl objects

This makes it possible for yosys commands to return values when invoked
as tcl commands. Right now no commands natively support this, but the
tee command can be used with json output like this:

```tcl
set stat [yosys tee -q -s result.json stat -json -top top]
dict get $stat modules \\top num_cells_by_type \$pmux
```

Or with newline separated lists like this:

```tcl
split [yosys tee -q -s result.string select -list top] "\n"
```

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