Completions should be prevented for single-line comments

[dalyIsaac]

Langium version: 4.0.0
Package name: langium

Original discussion: #2015

Steps To Reproduce

1. Create a grammar with a single-line comment as a hidden terminal
2. Create a completion provider
3. Start typing after a single-line comment has started

Link to code example: https://github.com/TypeFox/monaco-languageclient/blob/main/packages/examples/src/langium/statemachine/ls/statemachine.langium

Link to example: https://langium.org/showcase/statemachine/

The current behavior

Completions are currently possible within single-line comments.

The expected behavior

Completions should be able to be disabled within single-line comments.

There are currently tests for multi-line comments, but not for single-line comments:

langium/packages/langium/test/lsp/completion-provider.test.ts

Lines 339 to 424 in 910e8a0

	test('Can perform completion for fully qualified names', async () => {
	const grammar = `
	grammar FQNCompletionTest
	entry Model:
	(persons+=Person | greetings+=Greeting)*;
	Person:
	'person' name=FQN;
	Greeting:
	'Hello' person=[Person:FQN] '!';
	FQN returns string: ID ('.' ID)*;
	hidden terminal WS: /\\s+/;
	terminal ID: /[_a-zA-Z][\\w_]*/;
	hidden terminal ML_COMMENT: /\\/\\*[\\s\\S]*?\\*\\//;
	`;
	const services = await createServicesForGrammar({ grammar });
	const completion = expectCompletion(services);
	const text = `
	person John.Miller
	person John.Smith.Junior
	person John.Smith.Senior
	Hello <|>John<|>.Smi<|>th.Jun<|>ior
	Hello <|>John./* Hello */ <|>Miller
	`;
	await completion({
	text: text,
	index: 0,
	expectedItems: [
	'John.Miller',
	'John.Smith.Junior',
	'John.Smith.Senior'
	]
	});
	await completion({
	text: text,
	index: 1,
	expectedItems: [
	'John.Miller',
	'John.Smith.Junior',
	'John.Smith.Senior'
	]
	});
	await completion({
	text: text,
	index: 2,
	expectedItems: [
	'John.Smith.Junior',
	'John.Smith.Senior'
	]
	});
	await completion({
	text: text,
	index: 3,
	expectedItems: [
	'John.Smith.Junior'
	]
	});
	await completion({
	text: text,
	index: 4,
	expectedItems: [
	'John.Miller',
	'John.Smith.Junior',
	'John.Smith.Senior'
	]
	});
	// A comment within the FQN should prevent any completion from appearing
	await completion({
	text: text,
	index: 5,
	expectedItems: []
	});
	});