demos: rename demo_helpers.sh and demo_resolve_conflict.sh

I think it's clearer if only the actual demos started with `demo_`,
so I renamed `demo_helpers.sh` to just `helpers.sh`.

`demo_resolve_conflict.sh` should match `resolve_conflicts.png` (with an s).

demos/demo_git_compat.sh

@@ -1,6 +1,6 @@
 #!/bin/bash
 set -euo pipefail
-. "$(dirname "$0")"/demo_helpers.sh
+. "$(dirname "$0")"/helpers.sh
 
 new_tmp_dir
 

demos/demo_juggle_conflicts.sh

@@ -1,6 +1,6 @@
 #!/bin/bash
 set -euo pipefail
-. "$(dirname "$0")"/demo_helpers.sh
+. "$(dirname "$0")"/helpers.sh
 
 new_tmp_dir
 jj init --config-toml ui.allow-init-native=true

demos/demo_operation_log.sh

@@ -1,6 +1,6 @@
 #!/bin/bash
 set -euo pipefail
-. "$(dirname "$0")"/demo_helpers.sh
+. "$(dirname "$0")"/helpers.sh
 
 new_tmp_dir
 jj git clone https://github.com/octocat/Hello-World

demos/demo_resolve_conflicts.sh

@@ -1,6 +1,6 @@
 #!/bin/bash
 set -euo pipefail
-. "$(dirname "$0")"/demo_helpers.sh
+. "$(dirname "$0")"/helpers.sh
 
 new_tmp_dir
 jj git clone https://github.com/octocat/Hello-World

demos/demo_working_copy.sh

@@ -1,6 +1,6 @@
 #!/bin/bash
 set -euo pipefail
-. "$(dirname "$0")"/demo_helpers.sh
+. "$(dirname "$0")"/helpers.sh
 
 new_tmp_dir
 jj git clone https://github.com/octocat/Hello-World