feat: custom starter message configuration

[abeatrix]

RE: https://sourcegraph.slack.com/archives/C05AGQYD528/p1693983800785199

This PR adds a new configuration cody.chat.preInstruction to allow setting a custom starter message that will prepend chat requests to Cody:

• Configuration now has a cody.chat.preInstruction string property.
• A new setting cody.custom.starter was added in vscode/package.json to allow configuring a custom message.
• getConfiguration() was updated to read the new customStarter config value.
• ConfigurationWithAccessToken gained a customStarter property.
• main.ts imports PromptMixin to set the custom starter message on extension start.

Test plan

1. set cody.chat.preInstruction in your user config, e.g. "call me jeff"
2. reload extension
3. submit a chat request
4. check the output channel to see your setting was added to the chat request

[valerybugakov]

Many settings are grouped under feature categories (e.g., cody.autocomplete.* or cody.inlineChat.*). Would it make sense to apply this pattern here too?

[toolmantim]

What about cody.chat.preInstruction with the description "An instruction to be included at the start of all chat messages sent to Cody"

That is assuming it's chat messages only?

[abeatrix]

Yea autocompletes uses a different prompt so this is only for chat

[toolmantim]

Great! I left a suggestion for a new config name.