feat: Enable use of latest: in .tool-versions files

This change enables `asdf`'s existing latest-version-resolution
functionality within the `.tool-versions` file itself, ie rather
than having to have a `.tool-versions` file that contains a full
version number:

```
java corretto-21.0.5.11.1
```

...you can now use the same `latest:` syntax that is already available
in the `local` & `global` commands, ie:

```
java latest:corretto-21
```

Currently, using `latew` `asdf local python latest:3.7`

### Implementation

A new `resolve_version_spec()` function has been extracted from the
existing `version_command()` function. This takes a version-spec string,
like `latest:corretto-11` or `corretto-21.0.5.11.1`, and resolves it to
a definite installed version number (if the resolved version is not
installed, the appropriate error message is shown).

This new `resolve_version_spec()` function is now called in
`select_version()`, used by the `with_shim_executable()` function,
meaning that any execution of the `asdf` shim (eg, executing `java`)
will now resolve any version specifications found in the `.tool-versions`
file - if `.tool-versions` contains `java latest:corretto-21`, this
will be resolved and the latest version of Java 21 used.

## Other Information

Previous PRs relating to `latest`:

* asdf-vm#575 in November 2019: added the `latest`
  command, eg `asdf latest python 3.6` reports the latest version of Python 3.6.
* asdf-vm#633 in July 2021: made it possible
  to specify `latest` when using the `local` & `global` commands, eg:
  `asdf local python latest:3.7` - this would save a precise version number
  to `.tools-versions`, which is undesired behaviour for us at the Guardian.

bin/asdf

@@ -7,6 +7,8 @@ fi
 
 # shellcheck source=lib/utils.bash
 . "$(dirname "$(dirname "$0")")/lib/utils.bash"
+# shellcheck source=lib/functions/versions.bash
+. "$(dirname "$(dirname "$0")")/lib/functions/versions.bash"
 
 find_cmd() {
   local cmd_dir="$1"

lib/functions/versions.bash

@@ -37,27 +37,7 @@ version_command() {
   declare -a resolved_versions
   local item
   for item in "${!versions[@]}"; do
-    IFS=':' read -r -a version_info <<<"${versions[$item]}"
-    if [ "${version_info[0]}" = "latest" ] && [ -n "${version_info[1]}" ]; then
-      version=$(latest_command "$plugin_name" "${version_info[1]}")
-    elif [ "${version_info[0]}" = "latest" ] && [ -z "${version_info[1]}" ]; then
-      version=$(latest_command "$plugin_name")
-    else
-      # if branch handles ref: || path: || normal versions
-      version="${versions[$item]}"
-    fi
-
-    # check_if_version_exists should probably handle if either param is empty string
-    if [ -z "$version" ]; then
-      exit 1
-    fi
-
-    if ! (check_if_version_exists "$plugin_name" "$version"); then
-      version_not_installed_text "$plugin_name" "$version" 1>&2
-      exit 1
-    fi
-
-    resolved_versions+=("$version")
+    resolved_versions+=("$(resolve_version_spec "${versions[$item]}")")
   done
 
   if [ -f "$file" ] && grep -q "^$plugin_name " "$file"; then
@@ -79,6 +59,32 @@ version_command() {
   fi
 }
 
+resolve_version_spec() {
+  local version_spec=$1
+
+  IFS=':' read -r -a version_info <<<"$version_spec"
+  if [ "${version_info[0]}" = "latest" ] && [ -n "${version_info[1]}" ]; then
+    version=$(latest_command "$plugin_name" "${version_info[1]}")
+  elif [ "${version_info[0]}" = "latest" ] && [ -z "${version_info[1]}" ]; then
+    version=$(latest_command "$plugin_name")
+  else
+    # if branch handles ref: || path: || normal versions
+    version="${versions[$item]}"
+  fi
+
+  # check_if_version_exists should probably handle if either param is empty string
+  if [ -z "$version" ]; then
+    exit 1
+  fi
+
+  if ! (check_if_version_exists "$plugin_name" "$version"); then
+    version_not_installed_text "$plugin_name" "$version" 1>&2
+    exit 1
+  fi
+
+  printf "%s\n" "$version"
+}
+
 list_all_command() {
   local plugin_name=$1
   local query=$2

lib/utils.bash

@@ -736,7 +736,8 @@ select_version() {
     version_and_path=$(find_versions "$plugin_name" "$search_path")
     IFS='|' read -r version_string _path <<<"$version_and_path"
     IFS=' ' read -r -a usable_plugin_versions <<<"$version_string"
-    for plugin_version in "${usable_plugin_versions[@]}"; do
+    for version_spec in "${usable_plugin_versions[@]}"; do
+      plugin_version="$(resolve_version_spec "$version_spec")"
       for plugin_and_version in "${shim_versions[@]}"; do
         local plugin_shim_name
         local plugin_shim_version