Remove useless FEAT_BROWSE ifdef

The only thing this affected was the return value of
has('browsefilter').

Before this change, building Neovim would recursively search parent
directories for a .git directory. If Neovim was downloaded as a tarball
(i.e. without a .git directory), but placed in a subdirectory of
a Git repository, this caused a CMake error. Such a situation could
occur when packaging Neovim, for example.

Unfortunately, the previous attempt in #3317 did not fix this problem.

* Hide commit information from --version if we can't find any (e.g. when
  building from tarball).

To define a release in CMake, set NVIM_VERSION_PRERELEASE to "".
This will modify --version output to:

    * Show annotated Git tag instead of commit hash (NVIM_VERSION_COMMIT).
    * Hide commit date (NVIM_VERSION_BUILD).

These are: Release, MinSizeRel, and RelWithDebInfo.

Closes #2913.

Introduce new build type Dev that replaces RelWithDebInfo for development
builds off master and has optimizations, debug info, and logging enabled.
Keep assertions enabled for RelWithDebInfo.

Update .gitignore