fetch-configlet: rename release assets

Before this commit, the release assets were named like:

    configlet-linux-64bit.tgz
    configlet-mac-64bit.tgz
    configlet-windows-64bit.zip
    configlet_4.0.0-beta.7_checksums_sha256.txt

With this commit, the next release will have assets named:

    configlet_4.0.0-beta.8_checksums_sha256.txt
    configlet_4.0.0-beta.8_linux_x86-64.tar.gz
    configlet_4.0.0-beta.8_macos_x86-64.tar.gz
    configlet_4.0.0-beta.8_windows_x86-64.zip

Where we:

- Make the archive naming format match that of the checksums file
- Add a version string
- Delimit with an underscore, not a hyphen
- Rename `64bit` to `x86-64`
- Rename `32bit` to `i386`
- Rename `tgz` to `tar.gz`
- Rename `mac` to `macos`

So far, we have only released configlet for x86-64, and the release
assets have always had an ambiguous `64bit` in the names. This naming
format was old (added by d4c6e26836a5, 2020-10-09), and was ultimately
inherited from:

- the `exercism/configlet-v2` releases [1]
- the `exercism/cli` releases [2][3]

However, we're getting closer to adding releases for other 64-bit
architectures, so it's especially important to change the names now.

The rationale for the particular use of underscores and hyphens is:

- We want to have the version string in the asset filenames

- Configlet uses version strings that are compatible with the Semantic
  Versioning spec

- It is more common to use hyphens in executable names than
  underscores

- The Semantic Versioning spec forbids using an underscore, and
  specifies that prerelease versions can use hyphens [4]:

      Section 9:

      A pre-release version MAY be denoted by appending a hyphen and a
      series of dot separated identifiers immediately following the patch
      version. Identifiers MUST comprise only ASCII alphanumerics and
      hyphens [0-9A-Za-z-].
      [...]
      Examples: 1.0.0-alpha, 1.0.0-alpha.1, 1.0.0-0.3.7, 1.0.0-x.7.z.92

That is, there's an argument for the format of:

    some-app_1.0.0-beta.1_linux_x86-64.tar.gz

rather than:

    some_app-1.0.0-beta.1-linux-x86_64.tar.gz

because `_` fully separates the components of the first, and `-` does
not separate the components of the second (due to the `-beta` part of
the version string). This does ignore the convention of `_` as an
inter-word space, however.

[1] https://github.com/exercism/v2-configlet/releases
[2] exercism/cli#700 (comment)
[3] https://github.com/exercism/cli/releases/tag/v3.0.12
[4] https://semver.org/

exercism/configlet#363
exercism/configlet#24
exercism/configlet#705

bin/fetch-configlet

@@ -24,12 +24,12 @@ get_download_url() {
   local latest='https://api.github.com/repos/exercism/configlet/releases/latest'
   local arch
   case "$(uname -m)" in
-    *64*)  arch='64bit' ;;
-    *686*) arch='32bit' ;;
-    *386*) arch='32bit' ;;
-    *)     arch='64bit' ;;
+    x86_64) arch='x86-64' ;;
+    *686*)  arch='i386'   ;;
+    *386*)  arch='i386'   ;;
+    *)      arch='x86-64' ;;
   esac
-  local suffix="${os}-${arch}.${ext}"
+  local suffix="${os}_${arch}.${ext}"
   curl "${curlopts[@]}" --header 'Accept: application/vnd.github.v3+json' "${latest}" |
     grep "\"browser_download_url\": \".*/download/.*/configlet.*${suffix}\"$" |
     cut -d'"' -f4
@@ -48,7 +48,7 @@ main() {
 
   local os
   case "$(uname)" in
-    Darwin*)   os='mac'     ;;
+    Darwin*)   os='macos'   ;;
     Linux*)    os='linux'   ;;
     Windows*)  os='windows' ;;
     MINGW*)    os='windows' ;;
@@ -58,8 +58,8 @@ main() {
 
   local ext
   case "${os}" in
-    windows*) ext='zip' ;;
-    *)        ext='tgz' ;;
+    windows*) ext='zip'    ;;
+    *)        ext='tar.gz' ;;
   esac
 
   echo "Fetching configlet..." >&2