@export doesn't work as expected based on @block syntax.

[chriseppstein]

1. @export foo from "./foo.block.css" should export the default block of foo.block.css as foo.
2. @export (bar) from "./foo.block.css" should export the bar block of foo.block.css as bar.
3. @export (bar as xyz) from "./foo.block.css" should export thebar block of foo.block.css as xyz.
4. @export abc should export the local block reference to abc as abc.
5. @export (abc as zyx) should export the local block reference to abc as zyx.

When there's parens, we should accept any number of comma delimited export expressions where any export expression can be an identifier or two identifiers joined by as.

When there's not parens, we should only accept a single block name.