Explicit emit output support

[wackywendell]

Summary

Add support for explicit emit output mapping in the text format.

Problem

The text format currently shows field references like $0, $1, $2 but doesn't support explicit emit mappings when output columns are reordered or filtered:

• There is no distinction between an output mapping that uses Emit but lists all columns and using Direct
• Certain relations like Project and Read may have expressions (Project) or columns (Read) that are not output and hidden; those shown are shown in output order, not the precise way they were put into the plan

See RelCommon.emit for the protobuf definition.

Goal

Enable explicit, precise specification of output column mappings in the text format.

Success Criteria

• Parser supports explicit emit syntax
• Textifier can optionally output explicit emit mappings
• Roundtrip testing works