Add SwiftFormat to the project. (#129)

* Run swiftformat for whitespace.
* Run swiftformat with some rules.

.swiftformat

@@ -0,0 +1,17 @@
+--swiftversion 5.6
+
+--exclude ElementX/Sources/Generated
+
+--disable wrapMultiLineStatementBraces
+--disable blankLinesAtStartOfScope
+--disable blankLinesAtEndOfScope
+--disable hoistPatternLet
+
+--commas inline
+--ifdef no-indent
+--nospaceoperators ...,..<
+--stripunusedargs closure-only
+--trimwhitespace nonblank-lines
+--wrapparameters after-first
+--redundanttype inferred
+--emptybraces spaced