write script to combine multiple LSIF dumps of the same repo locally before upload

[garobrik]

Users with distributed build systems want to run indexers on subsets of their repos on multiple machines, and then combine the resulting LSIF dumps. This could be something we do on the backend, but this issue is for a stopgap (or maybe ultimately simpler solution): write a script to combine them locally.

Some considerations:

• We will need to unique-ify IDs between dumps
• Dumps might have different project roots
• We should be able to lean on backend canonicalization somewhat but not sure how much

[github-actions]

Heads up @macraig - the "team/code-intelligence" label was applied to this issue.