[scss] rename not working for placeholder selectors

[QJjs]

• VSCode Version: 1.28.2
• OS Version: Windows 10

Note: this only happens for .scss file types, as .sass is not supported by VSCode without extensions.

Steps to Reproduce:

1. Place cursor over sass placeholder selector
2. Initiate "Rename Symbol" command (through command palette, keyboard shortcuts, etc.)
3. Attempt to rename

After completion, the text editor will have renamed the variable throughout the document but removed the leading percent sign in the process. Even this can be dealt with by including a leading percent sign while renaming, it should be considered an issue because when the user does not take the effort to work around this quirk, the behavior of the selector is changed.

Example:

%foo {
	...
}

.bar {
	@extends %foo;
}

Attempting to rename foo to foobar results with this:

foobar {
	...
}

.bar {
	@extends foobar;
}

Rather than expected:

%foobar {
	...
}

.bar {
	@extends %foobar;
}

Does this issue occur when all extensions are disabled?: Yes