Underscore does not reliably trigger autocomplete

[fongandrew]

_ does not seem to trigger as the trailing character in a class name (it works fine if you type _ in the middle of the class though).

Logging shows that the provideCompletionItems method here is not even called in that scenario:

css-class-autocomplete/src/extension.ts

Lines 200 to 203 in c51e55a

	async provideCompletionItems(
	document: vscode.TextDocument,
	position: vscode.Position,
	): Promise<vscode.CompletionItem[] | undefined> {

This suggests the issue is either in how we're configuring the extension (although _ is listed in the trigger chars, an issue in VSCode itself, or maybe conflicting with some other extension or setting?