Dart-sass

* Fix an edge case where slash-separated numbers were written to the stylesheet
with a slash even when they're used as part of another arithmetic operation,
such as being concatenated with a string.

* Don't put style rules inside empty `keyframes` selectors.

* Deprecate `!global` variable assignments to variables that aren't yet defined.
This deprecation message can be avoided by assigning variables to `null` at
the top level before globally assigning values to them.

Dart API

* Explicitly mark classes that were never intended to be subclassed or
implemented as "sealed".

* Properly quote attribute selector values that start with identifiers but end
with a non-identifier character.

* Improve error output, particularly for errors that cover multiple lines.

* Improve source locations for some parse errors. Rather than pointing to the
next token that wasn't what was expected, they point *after* the previous
token. This should generally provide more context for the syntax error.

* Produce a better error message for style rules that are missing the closing
`}`.

* Produce a better error message for style rules and property declarations
within `function` rules.

Command-Line Interface

* Passing a directory on the command line now compiles all Sass source files in
the directory to CSS files in the same directory, as though `dir:dir` were
passed instead of just `dir`.

* The new error output uses non-ASCII Unicode characters by default. Add a
`--no-unicode` flag to disable this.

* Fix a performance bug where stylesheet evaluation could take a very long time
when many binary operators were used in sequence.

* `rgb()` and `hsl()` now treat unquoted strings beginning with `env()`,
`min()`, and `max()` as special number strings like `calc()`.