Add theme-specific color customizations. Fix #36860

[usernamehw]

My attempt at implementing theme-scoped color customizations #36860 .

• IntelliSense in settings
• Colors update on settings change
• Colors update on theme change
• Priority scoped => customized => theme

---

Settings example:

"workbench.colorCustomizations": {
    "[Monokai Dimmed]": {
        "activityBar.background": "#66F",
    },
    "activityBar.background": "#ddd",
    "[Monokai]": {
        "activityBar.background": "#F66"
    }
},
"editor.tokenColorCustomizations": {
    "[Monokai Dimmed]": {
        "comments": "#66F",
    },
    "comments": "#ddd",
    "[Monokai]": {
        "textMateRules": [{
            "scope": "comment",
            "settings": {
                "foreground": "#F66"
            },   
        }]
    },
},

cc @aeschli

[aeschli]

@usernamehw Looks good!
Only suggestion is to improve the way how the schema is constructed. What do you think?

[usernamehw]

It now register configuration with properties as empty object so it doesn't have to write them twice.

In addition, it uses shallow copy instead of deep.

[usernamehw]

@aeschli I made some changes...

[aeschli]

Thanks a lot @usernamehw , great PR!