further clean up border code; update paragraph borders to use shared type; BREAKING: paragraph border value attr renamed to style, to match other usages of borders

src/file/table/table-cell/table-cell.spec.ts

@@ -1,7 +1,7 @@
 import { expect } from "chai";
 
 import { Formatter } from "export/formatter";
-import { BorderStyle } from "file/styles";
+import { BorderStyle } from "file/border";
 
 import { ShadingType } from "file/shading";
 import { TableCell } from "./table-cell";