Extract InlineCompletionProvider to its own crate (#20935)

This PR extracts the `InlineCompletionProvider` trait and its related
types out of `editor` and into a new `inline_completion` crate.

By doing so we're able to remove a dependency on `editor` from the
`copilot` and `supermaven` crates.

We did have to move `editor::Direction` into the `inline_completion`
crate, as it is referenced by the `InlineCompletionProvider`. This
should find a better home, at some point.

Release Notes:

- N/A

crates/copilot/src/copilot_completion_provider.rs

@@ -1,8 +1,8 @@
 use crate::{Completion, Copilot};
 use anyhow::Result;
 use client::telemetry::Telemetry;
-use editor::{CompletionProposal, Direction, InlayProposal, InlineCompletionProvider};
 use gpui::{AppContext, EntityId, Model, ModelContext, Task};
+use inline_completion::{CompletionProposal, Direction, InlayProposal, InlineCompletionProvider};
 use language::{
     language_settings::{all_language_settings, AllLanguageSettings},
     Buffer, OffsetRangeExt, ToOffset,