refactor(amazonq): expose lsp inline completion as an experiment

[jpinkney-aws]

Problem

• We previously thought inline completion was going to be launched first so we didn't end up putting it behind a feature flag

Solution

• Add inline completion behind a feature flag

---

• Treat all work as PUBLIC. Private feature/x branches will not be squash-merged at release time.
• Your code changes must meet the guidelines in CONTRIBUTING.md.
• License: I confirm that my contribution is made under the terms of the Apache 2.0 license.

[github-actions]

• This pull request modifies code in src/* but no tests were added/updated.
  • Confirm whether tests should be added or ensure the PR description explains why tests are not required.