feat: add warning theme variant for button

[FrediWa]

Description

For sake of consistency, this PR adds the warning variant for vaadin-button.

Fixes #5911

Type of change

• Feature

Checklist

• I have added tests to ensure my change is effective and works as intended.

[vursen]

Please add screenshot tests for this change. You can find an example here:

web-components/packages/button/test/visual/lumo/button.test.js

Lines 133 to 168 in 1cabf0c

	['contrast', 'success', 'error'].forEach((variant) => {
	describe(variant, () => {
	it('primary', async () => {
	element.setAttribute('theme', `primary ${variant}`);
	await visualDiff(div, `theme-primary-${variant}`);
	});
	it('secondary', async () => {
	element.setAttribute('theme', `${variant}`);
	await visualDiff(div, `theme-secondary-${variant}`);
	});
	it('tertiary', async () => {
	element.setAttribute('theme', `tertiary ${variant}`);
	await visualDiff(div, `theme-tertiary-${variant}`);
	});
	it('primary disabled', async () => {
	element.setAttribute('theme', `primary ${variant}`);
	element.disabled = true;
	await visualDiff(div, `theme-primary-${variant}-disabled`);
	});
	it('secondary disabled', async () => {
	element.setAttribute('theme', `${variant}`);
	element.disabled = true;
	await visualDiff(div, `theme-secondary-${variant}-disabled`);
	});
	it('tertiary disabled', async () => {
	element.setAttribute('theme', `tertiary ${variant}`);
	element.disabled = true;
	await visualDiff(div, `theme-tertiary-${variant}-disabled`);
	});
	});
	});

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
No data about Coverage
0.0% Duplication on New Code

See analysis details on SonarCloud

[vaadin-bot]

This ticket/PR has been released with Vaadin 24.5.0.alpha4 and is also targeting the upcoming stable 24.5.0 version.