Fix non-ASCII path:line:column navigation (#51238)

Closes #43329

## Summary
This fixes `path:line:column` navigation for files containing non-ASCII
text.

Before this change, open path flows were passing the external column
directly into `go_to_singleton_buffer_point`. That happened to work for
ASCII, but it was wrong for Unicode because external columns are
user-visible character positions while the editor buffer stores columns
as UTF-8 byte offsets.

This PR adds a shared text layer conversion for external row/column
coordinates and uses it in the affected open-path flows:
 - file finder navigation
 - recent project remote connection navigation
 - recent project remote server navigation

It also adds regression coverage for the Unicode case that originally
failed.

As a small - necessary - prerequisite, this also adds
`remote_connection` test support to `file_finder`'s dev-deps so the
local regression test can build and run on this branch. That follows the
same feature mismatch pattern previously fixed in #48280. I wasn't able
to locally verify the tests/etc. w/o the addition... so I've rolled it
into this PR. Tests are green.

The earlier attempt in #47093 was headed in the right direction, but it
did not land and did not include the final regression coverage requested
in review.

## Verification
 - `cargo fmt --all -- --check`
 - `./script/clippy -p text`
 - `./script/clippy -p file_finder`
 - `./script/clippy -p recent_projects`
 - `cargo test -p file_finder --lib --no-run`
- `cargo test -p file_finder
file_finder_tests::test_row_column_numbers_query_inside_file -- --exact`
- `cargo test -p file_finder
file_finder_tests::test_row_column_numbers_query_inside_unicode_file --
--exact`
- `cargo test -p text
tests::test_point_for_row_and_column_from_external_source -- --exact`

## Manual

I reproduced locally on my machine (macOS) w/ a stateless launch using a
Unicode file (Cyrillic)

Before:
 - `:1:5` landed too far left
 - `:1:10` landed around the 4th visible Cyrillic character

After:
 - `:1:5` lands after 4 visible characters / before the 5th
 - `:1:10` lands after 9 visible characters / before the 10th

Release Notes:
- Fixed `path:line:column` navigation so non-ASCII columns land on the
correct character.

---------

Co-authored-by: Kirill Bulatov <kirill@zed.dev>

crates/multi_buffer/src/multi_buffer_tests.rs

@@ -72,6 +72,30 @@ fn test_singleton(cx: &mut App) {
     assert_consistent_line_numbers(&snapshot);
 }
 
+#[gpui::test]
+fn test_buffer_point_to_anchor_at_end_of_singleton_buffer(cx: &mut App) {
+    let buffer = cx.new(|cx| Buffer::local("abc", cx));
+    let multibuffer = cx.new(|cx| MultiBuffer::singleton(buffer.clone(), cx));
+
+    let excerpt_id = multibuffer
+        .read(cx)
+        .excerpt_ids()
+        .into_iter()
+        .next()
+        .unwrap();
+    let anchor = multibuffer
+        .read(cx)
+        .buffer_point_to_anchor(&buffer, Point::new(0, 3), cx);
+
+    assert_eq!(
+        anchor,
+        Some(Anchor::in_buffer(
+            excerpt_id,
+            buffer.read(cx).snapshot().anchor_after(Point::new(0, 3)),
+        ))
+    );
+}
+
 #[gpui::test]
 fn test_remote(cx: &mut App) {
     let host_buffer = cx.new(|cx| Buffer::local("a", cx));