Add language_models crate to house language model providers (#20945)

This PR adds a new `language_models` crate to house the various language
model providers.

By extracting the provider definitions out of `language_model`, we're
able to remove `language_model`'s dependency on `editor`, which improves
incremental compilation when changing `editor`.

Release Notes:

- N/A

crates/assistant/src/context.rs

@@ -25,13 +25,15 @@ use gpui::{
 
 use language::{AnchorRangeExt, Bias, Buffer, LanguageRegistry, OffsetRangeExt, Point, ToOffset};
 use language_model::{
-    logging::report_assistant_event,
-    provider::cloud::{MaxMonthlySpendReachedError, PaymentRequiredError},
     LanguageModel, LanguageModelCacheConfiguration, LanguageModelCompletionEvent,
     LanguageModelImage, LanguageModelRegistry, LanguageModelRequest, LanguageModelRequestMessage,
     LanguageModelRequestTool, LanguageModelToolResult, LanguageModelToolUse, MessageContent, Role,
     StopReason,
 };
+use language_models::{
+    provider::cloud::{MaxMonthlySpendReachedError, PaymentRequiredError},
+    report_assistant_event,
+};
 use open_ai::Model as OpenAiModel;
 use paths::contexts_dir;
 use project::Project;