Add scalafmt

[lqhuang]

Nothing changes besides codes been formatted.

I copied .scalafmt from mill and did very little tunings for consistency and readability.

My local test results

$ ./mill -i __.publishArtifacts + __.test

[418/420] requests[2.13.15].test.test
[418] 
[420/420] requests[3.3.4].test.test
[420] 
[417/420] requests[2.12.20].test.test
[417] 
[420/420] ==================================================================== __.publishArtifacts + __.test ======================================================================= 1s

My local binary compatibility check results

$ ./mill -i __.mimaReportBinaryIssues
[build.mill-64/68] compile
[build.mill-64] [info] compiling 1 Scala source to /home/lqhuang/Git/requests-scala/out/mill-build/compile.dest/classes ...
[build.mill-64] [info] done compiling
[211/212] requests.jvm[2.12.20].mimaReportBinaryIssues
[210/212] requests.jvm[2.13.15].mimaReportBinaryIssues
[212/212] requests.jvm[3.3.4].mimaReportBinaryIssues
[211] Scanning binary compatibility in /home/lqhuang/Git/requests-scala/out/requests/jvm/2.12.20/compile.dest/classes ...
[210] Scanning binary compatibility in /home/lqhuang/Git/requests-scala/out/requests/jvm/2.13.15/compile.dest/classes ...
[212] Scanning binary compatibility in /home/lqhuang/Git/requests-scala/out/requests/jvm/3.3.4/compile.dest/classes ...
[210] Binary compatibility check passed
[211] Binary compatibility check passed
[212] Binary compatibility check passed
[212/212] ============================== __.mimaReportBinaryIssues ============================== 8s

[lqhuang]

@lihaoyi Already rebased from current master. All checks have passed. Also ready to merge. Any style feedback is welcome.

Regards

[lihaoyi]

thanks!