Expand support for TextMate-style scopes

[oleks]

Currently, we only distinguish between the following scopes:

• comment
• constant.character
• entity.name.type
• keyword
• string.quoted

For better support of editors with TextMate-style syntax-highlighting (e.g., Atom, vscode), we should probably segregate constants and keywords a little so that richer scope selectors can be applied.

Here are some scope selectors that could be relevant in Emerald, right off the bat:

• comment.line (probably better than comment)
• constant.numeric
• constant.character
• constant.character.escape
• constant.language (e.g., true)
• keyword.control (e.g., for)
• keyword.operator (e.g., as)
• storage.type (e.g., const)
• storage.modifier (e.g., attached)
• string.quoted.double (probably better than string.quoted)
• variable.language (e.g., self)