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

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 the`bar` 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.