feat: has('terminfo')

Problem:
No way to detect at runtime if the build includes unibilium (or whatever
terminfo layer we swap it with later).

Solution:
Support `has('terminfo')`.

.github/workflows/build.yml

@@ -55,7 +55,7 @@ jobs:
         run: make install
 
   use-existing-src:
-    name: Test USE_EXISTING_SRC_DIR=ON builds with no network access
+    name: Test offline build (USE_EXISTING_SRC_DIR=ON builds with no network access)
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v6

.github/workflows/test.yml

@@ -176,7 +176,7 @@ jobs:
       - name: Install
         run: |
           cmake --install build
-          "$INSTALL_PREFIX/bin/nvim" --version
+          "$INSTALL_PREFIX/bin/nvim" -V1 --version
           if ! "$INSTALL_PREFIX/bin/nvim" -u NONE -e -c ':help' -c ':qall'; then
             echo "Running ':help' in the installed nvim failed."
             echo "Maybe the helptags have not been generated properly."