Update semantic token mappings

[ijklam]

Make semantic tokens with modifier can map more specifically.

F# grammar	Sematic token	Corresponding TextMate rule scope
Values
Value	variable	variable.other.readonly.fsharp
Literal	variable.readonly	variable.other.constant.fsharp
Mutable value	variable.mutable	entity.name.function.mutable_variable.fsharp
Disposible value	variable.disposable	entity.name.function.disposable_variable.fsharp
Property
Object property	property	variable.other.property.may_mutable.fsharp
Record field	property.readonly	variable.other.constant.property.fsharp
Mutable record field	property.mutable	entity.name.function.mutable_property.fsharp
Method
Extension method	method.extension	entity.name.function.extension_method.fsharp

Here I set methods to bold, and properties to bold + italic. Let's see some pictures.
Before:

With this and ionide/FsAutoComplete#1434

User can custom their own style in settings.json using editor.tokenColorCustomizations. For example

  "editor.tokenColorCustomizations": {
    "textMateRules": [
      {
        "scope": "entity.name.function.disposable_variable.fsharp",
        "settings": {
          "foreground": "#009688"
        }
      },
    ]
  }

We can get

[TheAngryByrd]

This looks great. Thank you!