Adds an experimental command /symf that provides an indexed keyword search capability

[beyang]

Adds an experimental /symf command to the Cody sidebar. /symf uses symf, a CLI that implements LLM-enabled keyword search. MVP for #583

Use

1. Install symf: go get github.com/sourcegraph/symf/cmd/symf@latest
2. Set the following fields in the VS Code settings:

   "cody.experimental.symf.path": "the full path to the symf binary",
   "cody.experimental.symf.anthropicKey": "an api key from anthropic",
   

3. Type "/symf <your query>" in the Cody sidebar:
   

The first time /symf is used for a given repository, it will need to build an index, so you'll see a message like this:

An alert will appear when the index is finished building:

Comments

The command is experimental and requires manual steps (installing the binary and setting the config fields) at the moment. It's intended for internal use to test out the quality of the search results within the Cody UI.

Currently, the following languages are supported by symf:

• TypeScript
• TSX
• Go
• Python
• C

The next milestone is to make this a viable replacement for /search. This will require:

• Replacing the direct Anthropic dependency with Cody Gateway
• Auto-download the symf binary
• Show the expanded keyword query being used under the hood (more transparency)
• A better UX for communicating index build status
• (Potential) implement index-less search in symf
• Add more languages to symf

Test plan

Command is experimental only at the moment.

[sqs]

haven't run it, but i don't see anything that would cause the non-exoerimental functionality in the extension to fail, so lgtm to merge