FR: workspace-specific configuration

**What problem does this solve?**
Right now, we have a workspace-specific dictionary, and the global configuration of the Harper LSP. However, there is no way of configuring harper on a per-workspace basis. Use cases include:
- disable rules only in a specific project
- ignore files only in a specific project
- share config files between editors (e.g., VSCode & nvim)
- share config files with collaborators
- version control the config with the project, not with your dotfiles
- allow contributors to use the same harper config.

**Proposed Solution**
A project specific configuration, e.g., `harperrc` or `harper.json` or `harper.yaml`

**Examples**
Many other linters do have a file like this, e.g., `typos`, `biome` or `ruff`.

**Component**
- [x] LSP
- [x] CLI

**Additional Context**
#1733 describes a similar issue, but was closed by the stalebot, even though it would indeed be useful.

The issue names no files, tests, or entry points. Start by defining the workspace configuration format and how it should be discovered and shared by the LSP and CLI. Done means project-specific rules, ignored files, and shared configuration work consistently for both components.

Written by the indexing model from the issue text.