fix: zsh import multiline issue (#2799)

Here is my case:
```sh
curl -X POST https://example.com/api \
  -H "Content-Type: application/json" \
  -d '{
    "username": "name",
    "password": "pass",
    "details": {
      "age": 30,
      "location": "Earth"
    }
  }'
```

Using current release version i got:
```sh
curl -X POST https://example.com/api \\
  -H "Content-Type: application/json" \\
  -d '{\
    "username": "name",\
    "password": "pass",\
    "details": {\
      "age": 30,\
      "location": "Earth"\
    }\
  }'
```

and i test with `echo \\`
(https://github.com/atuinsh/atuin/pull/100#issuecomment-836477081) it
also works;

there is similar PR https://github.com/atuinsh/atuin/pull/2390, it only
works on multiline with `\` suffix, in my case i got:
```sh
curl -X POST https://example.com/api \
  -H "Content-Type: application/json" \
  -d '{\
```

## Checks
- [x] I am happy for maintainers to push small adjustments to this PR,
to speed up the review cycle
- [ ] I have checked that there are no existing pull requests for the
same thing

1 files changed, 2 insertions, 2 deletions

diff --git a/crates/atuin-client/src/import/zsh.rs b/crates/atuin-client/src/import/zsh.rs
index b65e2608..11e2f371 100644
--- a/crates/atuin-client/src/import/zsh.rs
+++ b/crates/atuin-client/src/import/zsh.rs
@@ -70,7 +70,7 @@ impl Importer for Zsh {
 
             if let Some(s) = s.strip_suffix('\\') {
                 line.push_str(s);
-                line.push_str("\\\n");
+                line.push('\n');
             } else {
                 line.push_str(&s);
                 let command = std::mem::take(&mut line);
@@ -188,7 +188,7 @@ mod test {
     #[tokio::test]
     async fn test_parse_file() {
         let bytes = r": 1613322469:0;cargo install atuin
-: 1613322469:10;cargo install atuin; \
+: 1613322469:10;cargo install atuin; \\
 cargo update
 : 1613322469:10;cargo :b̷i̶t̴r̵o̴t̴ ̵i̷s̴ ̷r̶e̵a̸l̷
 "