Switch from `CURSORLESS_TEST` to `CURSORLESS_MODE` environment variable

[pokey]

It can be either "test", "develop", or unset. If it is unset, it means production, because we don't have an easy way to set it in production and prob best not to have extra env vars in production just in case

• [ ] We should also try to avoid using extension.runMode
• [ ] We could also add an assert that extension.runMode is equal to CURSORLESS_MODE for sanity checking

Note that this is how it is done in our neovim support

See also https://github.com/cursorless-dev/cursorless/pull/2301