Add alacritty

Default linux terminal isn't great, and all the others don't seem to
work well. Let's use this instead.

Bootstrapfile

@@ -14,6 +14,7 @@ gui do
   install "localsend"
   install "pinta"
   install "xournalpp"
+  install "alacritty"
 
   macos do
     install "iterm2"

bootstrap/lib/bootstrap/package/sources.yml

@@ -158,3 +158,7 @@ xournalpp:
     name: Xournal++.Xournal++
   macos:
     name: xournal++
+
+alacritty:
+  ubuntu:
+    snap: true

test/bootstrap/fedora_test.rb

@@ -112,6 +112,10 @@ class FedoraTest < TestCase
       test "installs xournalpp" do
         assert_dnf_installed("xournalpp")
       end
+
+      test "installs alacritty" do
+        assert_dnf_installed("alacritty")
+      end
     else
       test "does not install flux" do
         assert_dnf_not_installed("fluxgui")
@@ -161,9 +165,13 @@ class FedoraTest < TestCase
         assert_dnf_not_installed("pinta")
       end
 
-      test "installs xournalpp" do
+      test "does not install xournalpp" do
         assert_dnf_not_installed("xournalpp")
       end
+
+      test "does not install alacritty" do
+        assert_dnf_not_installed("alacritty")
+      end
     end
 
     private

test/bootstrap/macos_test.rb

@@ -120,6 +120,10 @@ class MacosTest < TestCase
       test "installs xournalpp" do
         assert_brew_cask_installed("xournal++")
       end
+
+      test "installs alacritty" do
+        assert_brew_cask_installed("alacritty")
+      end
     else
       test "does not install iterm2" do
         assert_brew_cask_not_installed("iterm2")
@@ -176,6 +180,10 @@ class MacosTest < TestCase
       test "does not install xournalpp" do
         assert_brew_cask_not_installed("xournal++")
       end
+
+      test "does not install alacritty" do
+        assert_brew_cask_not_installed("alacritty")
+      end
     end
 
     private

test/bootstrap/ubuntu_test.rb

@@ -112,6 +112,10 @@ class UbuntuTest < TestCase
       test "installs xournalpp" do
         assert_apt_installed("xournalpp")
       end
+
+      test "installs alacritty" do
+        assert_snap_installed("alacritty")
+      end
     else
       test "does not install flux" do
         assert_apt_not_installed("fluxgui")
@@ -164,6 +168,10 @@ class UbuntuTest < TestCase
       test "does not install xournalpp" do
         assert_apt_not_installed("xournalpp")
       end
+
+      test "does not install alacritty" do
+        assert_snap_not_installed("alacritty")
+      end
     end
 
     private

test/bootstrap/windows_test.rb

@@ -76,6 +76,10 @@ class WindowsTest < TestCase
       assert_winget_installed("Xournal++.Xournal++")
     end
 
+    test "installs alacritty" do
+      assert_winget_installed("alacritty")
+    end
+
     private
 
     def assert_winget_installed(package_name)