Add compatibility note about Nu version (#1057)

This is my response for #1051 

The syntax of this note is inspired by Rust _Cargo.toml_ and Python
_pyproject.toml_.

custom-completions/git/git-completions.nu

@@ -1,3 +1,5 @@
+# nu-version: 0.102.0
+
 module git-completion-utils {
   export const GIT_SKIPABLE_FLAGS = ['-v', '--version', '-h', '--help', '-p', '--paginate', '-P', '--no-pager', '--no-replace-objects', '--bare']
 
@@ -106,8 +108,8 @@ module git-completion-utils {
     let floating_remotes = $lines | filter { "\t" not-in $in and $in not-in $tracked_remotes }
     $floating_remotes | each {
       let v = $in | split row -n 2 '/' | get 1
-      if $v != $current { [$v] } else []
+      if $v == $current { null } else $v
-    } | flatten
+    }
   }
 
   export def extract-mergable-sources [current: string]: list<string> -> list<record<value: string, description: string>> {

custom-completions/ssh/ssh-completions.nu

@@ -1,3 +1,5 @@
+# nu-version: 0.102.0
+
 export extern "ssh" [
     destination?: string@"nu-complete ssh-host"
     -4            # Forces ssh to use IPv4 addresses only.
@@ -41,11 +43,12 @@ module ssh-completion-utils {
         # │ 4 │                              │
         # ╰───┴──────────────────────────────╯
         let host = $in
-        let name = $host | get 0 | str trim | split row -r '\s+' | get 1
+        let $first_line = try { $host | first | str trim } catch { null }
         # Don't accept blocks like "Host *"
-        if ('*' in $name) {
+        if ($first_line | is-empty) or '*' in $first_line {
             null
         } else {
+            let name = $first_line |  split row -r '\s+' | get 1
             # May not contain hostname
             match ($host | slice 1.. | find -ir '^\s*Hostname\s') {
                 [] => null,