symf: support cancelling index

[beyang]

Fixes #1876

Follow-up tasks

• Known improvement opportunity: If a user triggers indexing when an index already exists and then cancels the new indexing, the old index is removed. It would be nicer if we could still reuse the old index. Will fix this in a follow-up PR.

Test plan

• rm -rf ~/.cody-symf to erase the symf index
• Open the cody repository
• Do a natural language search to trigger indexing
• Cancel the search. In the terminal, run ps aux | grep symf to verify the process was killed.
• Verify the index was not built by running find ~/.cody-symf/. You should only see contents in the .tmp and .failed directories.
• Try doing another search. The index should build this time.
• Refresh the index using the refresh button in the side panel. Cancel the reindex. Verify the index doesn't exist.

[dominiccooney]

Looks good!

[toolmantim]

Nice!

[abeatrix]

Let's go!

[kalanchan]

GOGOGO nice work!