Simplify Autocomplete testing using MockTextEditor

[hassoncs]

Introduces MockTextDocument and MockTextEditor classes to make creating documents for tests easier!

The previous tests were tricky to maintain because it's hard to visualize where the cursor is in a given document. These new test utils should make that easier going forward!

From this:

		const mockDocument = {
			lineAt: vi.fn().mockReturnValue({
				text: "    ", // whitespace line
			}),
			getText: vi.fn().mockReturnValue(""),
		}
		const mockPosition = new vscode.Position(0, 3) // Cursor inside a whitespace line

To this (all in one!):

const mockEditor = MockTextEditor.create(` ␣ `)
const mockPosition = mockEditor.selection.active;
const mockDocument = mockEditor.document;

The AutocompleteProvider.test.ts file is updated to leverage these new mock classes, replacing manual mocking of vscode.Document and vscode.Position objects. This change specifically improves the clarity and maintainability of tests related to whitespace handling and cursor positioning.

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[markijbema]

I like it!

I have one option which might make it slightly more flexible:

MockTextEditor.create((cursor) => ` {cursor} `)

that way we might also mock selections and such by enabling more parameters; but it is slightly more noisy. Totally up to you and @catrielmuller