feat(theme): add t.strong to existing style to cope with emboldened text (e.g. **bold** in GitHub markdown) (#587)

Co-authored-by: cleverlight <cleverlight@users.noreply.github.com>

Author: cleverlight

Diff for: themes/github/src/index.ts

@@ -29,7 +29,7 @@ export const githubLightInit = (options?: Partial<CreateThemeOptions>) => {
       { tag: [t.keyword, t.typeName, t.typeOperator, t.typeName], color: '#d73a49' },
       { tag: [t.string, t.meta, t.regexp], color: '#032f62' },
       { tag: [t.name, t.quote], color: '#22863a' },
-      { tag: [t.heading], color: '#24292e', fontWeight: 'bold' },
+      { tag: [t.heading, t.strong], color: '#24292e', fontWeight: 'bold' },
       { tag: [t.emphasis], color: '#24292e', fontStyle: 'italic' },
       { tag: [t.deleted], color: '#b31d28', backgroundColor: 'ffeef0' },
       { tag: [t.atom, t.bool, t.special(t.variableName)], color: '#e36209' },
@@ -69,7 +69,7 @@ export const githubDarkInit = (options?: Partial<CreateThemeOptions>) => {
       { tag: [t.keyword, t.typeName, t.typeOperator, t.typeName], color: '#ff7b72' },
       { tag: [t.string, t.meta, t.regexp], color: '#a5d6ff' },
       { tag: [t.name, t.quote], color: '#7ee787' },
-      { tag: [t.heading], color: '#d2a8ff', fontWeight: 'bold' },
+      { tag: [t.heading, t.strong], color: '#d2a8ff', fontWeight: 'bold' },
       { tag: [t.emphasis], color: '#d2a8ff', fontStyle: 'italic' },
       { tag: [t.deleted], color: '#ffdcd7', backgroundColor: 'ffeef0' },
       { tag: [t.atom, t.bool, t.special(t.variableName)], color: '#ffab70' },