feat: support mermaid code blocks in Markdown (#7490)

Co-authored-by: Joshua Chen <sidachen2003@gmail.com>
Co-authored-by: sebastienlorber <lorber.sebastien@gmail.com>

packages/docusaurus-theme-mermaid/src/__tests__/validateThemeConfig.test.ts

@@ -0,0 +1,75 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import {
+  validateThemeConfig,
+  DEFAULT_THEME_CONFIG,
+} from '../validateThemeConfig';
+import type {Joi} from '@docusaurus/utils-validation';
+import type {ThemeConfig, UserThemeConfig} from '@docusaurus/theme-mermaid';
+
+function testValidateThemeConfig(themeConfig: UserThemeConfig) {
+  function validate(
+    schema: Joi.ObjectSchema<ThemeConfig>,
+    cfg: UserThemeConfig,
+  ) {
+    const {value, error} = schema.validate(cfg, {
+      convert: false,
+    });
+    if (error) {
+      throw error;
+    }
+    return value;
+  }
+
+  return validateThemeConfig({validate, themeConfig});
+}
+
+describe('validateThemeConfig', () => {
+  it('undefined config', () => {
+    const mermaid = undefined;
+    expect(testValidateThemeConfig({mermaid})).toEqual(DEFAULT_THEME_CONFIG);
+  });
+
+  it('nonexistent config', () => {
+    expect(testValidateThemeConfig({})).toEqual(DEFAULT_THEME_CONFIG);
+  });
+
+  it('empty config', () => {
+    const mermaid = {};
+    expect(testValidateThemeConfig({mermaid})).toEqual(DEFAULT_THEME_CONFIG);
+  });
+
+  it('theme', () => {
+    const mermaid = {
+      theme: {
+        light: 'light',
+        dark: 'dark',
+      },
+    };
+    expect(testValidateThemeConfig({mermaid})).toEqual({
+      mermaid: {
+        ...DEFAULT_THEME_CONFIG.mermaid,
+        ...mermaid,
+      },
+    });
+  });
+
+  it('mermaid options', () => {
+    const mermaid = {
+      options: {
+        fontFamily: 'Ariel',
+      },
+    };
+    expect(testValidateThemeConfig({mermaid})).toEqual({
+      mermaid: {
+        ...DEFAULT_THEME_CONFIG.mermaid,
+        ...mermaid,
+      },
+    });
+  });
+});