Add preliminary .clang-format file.

Our current coding style is not very uniform and we don't seem to be
following any particular standard, even though there are some patterns
we more or less follow.

This attempts to mostly reproduce our current style. However, it still
introduces quite a lot of changes since clang-format doesn't yet support
dangling parentheses[*] (where the closing parenthesis of a parameter
list goes on its own line).

[*]: https://reviews.llvm.org/D33029
5 jobs for wip-clang-format in 2 minutes and 17 seconds (queued for 21 seconds)
Status Name Job ID Coverage
  Build
passed build:lib #21561
docker

00:01:25

 
  Test
passed test:lib #21562
docker

00:00:51

 
  Trigger
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passed trigger:javascript

passed trigger:python