Mark the schema of our built-in extension's snippets

[mjbvz]

Adds "$schema": "vscode://schemas/snippets" to all the snippet files for our built-in snippets. This gives us intellisense while editing the snippets

@aeschli Adding the schema works but I see a yellow warning on the $schema:

I cannot figure out what this error means or how to fix it. Could you please take a look?

[aeschli]

The 'object expected ' warning comes from here:

vscode/src/vs/workbench/contrib/snippets/browser/snippets.contribution.ts

Line 57 in 5c9a1b5

additionalProperties: {

Every property in the snippet file is considered a snippet declaration

[aeschli]

Instead of adding "$schema": "vscode://schemas/snippets" everywhere, I would suggest a specific file name that we can the associate to.

foo.code-snippets.json
Going for code-snippets instead of snippets avoid clashing with other (non- VSCode) snippet files.

I will use the same name convention in the yeoman generator

[aeschli]

We already have a suffix .code-snippets for global snippets. Global snippets are snippets that also support the scope property
Talked to @jrieken and it turns out also extension snippets are allowed to use the scope property.
-> We use to use .code-snippets as suffix for extension snippets.