Custom modes should have a `name` attribute in frontmatter

[pierceboggan]

Rather than relying on the file name in front of .chatmode.md to determine the mode name, we should have an attribute in the frontmatter to specify a name:

---
name: 'Planning'
description: 'Implement a feature based on a specification and implementation plan.'
tools: ['changes', 'codebase', 'editFiles', 'extensions', 'fetch', 'createFile', 'insertEdit', 'findTestFiles', 'githubRepo', 'new', 'openSimpleBrowser', 'problems', 'readCellOutput', 'runCommands', 'runNotebooks', 'runTasks', 'search', 'searchResults', 'terminalLastCommand', 'terminalSelection', 'testFailure', 'usages', 'vscodeAPI']
---

Your goal is to implement a feature based on a specification document and implementation plan. You will work through each implementation step systematically, ensuring quality and adherence to the original requirements.

This avoids situations in source where file names are commonly lowercase from appearing strangely in the modes dropdown. From a user: