Add a Extension Config API

[michaelgruenewald]

This adds an exported API for other extensions to add rust-analyzer-specific configurations for #18476. It provides a simple API where an extension can just call something like

const rustAnalyzer = await vscode.extensions.getExtension("rust-lang.rust-analyzer")?.activate();
await rustAnalyzer?.addConfiguration(ctx.extension.id, {
    "cargo.cfgs": ["foo", "bar"],
});

to configure aspects. The extension configurations are persisted in the workspace, but only configurations from extensions are are actually present are used, so that uninstalled extensions, etc. aren't causing any trouble.

This does pull in lodash as a dependency, but IMO this is neither very heavy nor critical, and it was already in the dev closure anyway.

Also cf. the discussion in https://rust-lang.zulipchat.com/#narrow/channel/185405-t-compiler.2Frust-analyzer/topic/VSCode.20extension.20config.20feature.20contribution

Fixes #18476