Title tab color should dim on unfocused tab

### Description of the new feature

Today, when you have a unfocused tab, the background changes, but not the color of the title. If the focused and unfocused colors are almost the same, you have problems to see which one is focused.

(first is focused)

Theme info:
```
"name": "Dark+",
"tab":
{
"background": "terminalBackground",
"iconStyle": "default",
"showCloseButton": "never",
"unfocusedBackground": "#00000000"
},
"tabRow":
{
"background": "#333333FF",
"unfocusedBackground": "#D4D4D4FF"
},
"window":
{
"applicationTheme": "dark",
"experimental.rainbowFrame": false,
"frame": null,
"unfocusedFrame": null,
"useMica": false
}
```

Explorer @ Win 11 does this:

### Proposed technical implementation details

Let the title text to be customized, like the `unfocusedBackground`, but for text title (`unfocusedTabTitle`, maybe?). Or, at least dim the title text too.

No source files, tests, or entry points are named. Start by locating the Windows Terminal tab title rendering and the existing tab or tabRow unfocused color handling, then verify the unfocused title is visibly dimmed without changing the focused title or breaking theme customization.

Written by the indexing model from the issue text.