feat(contrib): add bumpver tool to help with semantic version releases

Adds a simple go command-line tool `bumpver` that will help us update source and docs with semantic version numbers as described in its README.md. Skip to the bottom to ignore the vendored code and read the important stuff.

TESTING: You can follow the instructions in README.md and simply paste those commands in, then `git diff` the result (and `git stash` afterward).

[skip ci]