more stable autocomplete (#442)

Makes autocomplete not jitter as much as you type, with a cache and a
few other techniques to help increase the (1) cache hit rate and (2) VS
Code's ability to reuse and not invalidate existing completion results.

Why this change?

- Extract the non-VS Code-specific parts of `getInlineCompletions` so we
can separately test *it* and the `InlineCompletionItemProvider` logic.
- All test cases were ported to the new `getInlineCompletions` (and all
pass without modification in substance).
  - This also helps with extracting completions to the agent.
- Make the cache and RequestManager simpler and dumber by exploiting VS
Code's existing behavior (of showing w/o jitter a full-line inline
completion while it is refreshed in the background).
- Remove the existing cache entirely and replace it with a simpler cache
that just memoizes calls (inside RequestManager) and does not have any
other text logic.
- Make the RequestManager not need to support `logId` rewriting or
retesting caches for a document (see below).
- Separately handle the case of "typing the last suggestion" (e.g.,
typing as suggested by the ghost text) vs. the general case of cached
network requests. This case is the only one where we want to reuse the
previous `logId`, and we can do so simply instead of (if the 2 cases are
conflated) requiring the cache to support `logId` rewriting.

Examples of bugs or undesirable behaviors this fixes:

- Absolute minimum jitter when deleting or typing ghost text.
- As long as you are still typing ghost text exactly as suggested, it
won't replace it underneath you with a new completion.
- If you delete through ghost text past the initial trigger point, it
will always suggest something new.
- When you press delete on an empty but indented line with ghost text,
the ghost text remains in place (no jitter) until you type something
different.

## Test plan

Use completions and watch the logIds. Try typing or deleting over ghost
text.

lib/shared/src/configuration.ts

@@ -24,7 +24,6 @@ export interface Configuration {
         | 'unstable-azure-openai'
     autocompleteAdvancedServerEndpoint: string | null
     autocompleteAdvancedAccessToken: string | null
-    autocompleteAdvancedCache: boolean
     autocompleteAdvancedEmbeddings: boolean
     autocompleteExperimentalCompleteSuggestWidgetSelection?: boolean
     pluginsEnabled?: boolean

vscode/package.json

@@ -885,11 +885,6 @@
           "type": "string",
           "markdownDescription": "Overwrite the access token used for code autocomplete. This is only supported with a provider other than `anthropic`."
         },
-        "cody.autocomplete.advanced.cache": {
-          "type": "boolean",
-          "default": true,
-          "markdownDescription": "Enables caching of code autocomplete."
-        },
         "cody.autocomplete.advanced.embeddings": {
           "order": 99,
           "type": "boolean",