Auto merge of #126597 - estebank:unicode-output, r=fmease

Add Unicode block-drawing compiler output support

Add nightly-only theming support to rustc output using Unicode box
drawing characters instead of ASCII-art to draw the terminal UI.

In order to enable, the flags `-Zunstable-options=yes --error-format=human-unicode` must be passed in.

After:

```
error: foo
  ╭▸ test.rs:3:3
  │
3 │       X0 Y0 Z0
  │   ┌───╿──│──┘
  │  ┌│───│──┘
  │ ┏││━━━┙
  │ ┃││
4 │ ┃││   X1 Y1 Z1
5 │ ┃││   X2 Y2 Z2
  │ ┃│└────╿──│──┘ `Z` label
  │ ┃└─────│──┤
  │ ┗━━━━━━┥  `Y` is a good letter too
  │        `X` is a good letter
  ╰╴
note: bar
  ╭▸ test.rs:4:3
  │
4 │ ┏   X1 Y1 Z1
5 │ ┃   X2 Y2 Z2
6 │ ┃   X3 Y3 Z3
  │ ┗━━━━━━━━━━┛
  ├ note: bar
  ╰ note: baz
note: qux
  ╭▸ test.rs:4:3
  │
4 │   X1 Y1 Z1
  ╰╴  ━━━━━━━━
```

Before:

```
error: foo
 --> test.rs:3:3
  |
3 |       X0 Y0 Z0
  |    ___^__-__-
  |   |___|__|
  |  ||___|
  | |||
4 | |||   X1 Y1 Z1
5 | |||   X2 Y2 Z2
  | |||____^__-__- `Z` label
  | ||_____|__|
  | |______|  `Y` is a good letter too
  |        `X` is a good letter
  |
note: bar
 --> test.rs:4:3
  |
4 | /   X1 Y1 Z1
5 | |   X2 Y2 Z2
6 | |   X3 Y3 Z3
  | |__________^
  = note: bar
  = note: baz
note: qux
 --> test.rs:4:3
  |
4 |   X1 Y1 Z1
  |   ^^^^^^^^
```

After:

![rustc output with unicode box drawing characters](https://github.com/rust-lang/rust/assets/1606434/d210b79a-6579-4407-9706-ba8edc6e9f25)

Before:
![current rustc output with ASCII art](https://github.com/rust-lang/rust/assets/1606434/5aecccf8-a6ee-4469-8b39-72fb0d979a9f)

compiler/rustc_session/src/session.rs

@@ -16,7 +16,9 @@ use rustc_data_structures::sync::{
 };
 use rustc_errors::annotate_snippet_emitter_writer::AnnotateSnippetEmitter;
 use rustc_errors::codes::*;
-use rustc_errors::emitter::{DynEmitter, HumanEmitter, HumanReadableErrorType, stderr_destination};
+use rustc_errors::emitter::{
+    DynEmitter, HumanEmitter, HumanReadableErrorType, OutputTheme, stderr_destination,
+};
 use rustc_errors::json::JsonEmitter;
 use rustc_errors::registry::Registry;
 use rustc_errors::{
@@ -966,6 +968,11 @@ fn default_emitter(
                     .macro_backtrace(macro_backtrace)
                     .track_diagnostics(track_diagnostics)
                     .terminal_url(terminal_url)
+                    .theme(if let HumanReadableErrorType::Unicode = kind {
+                        OutputTheme::Unicode
+                    } else {
+                        OutputTheme::Ascii
+                    })
                     .ignored_directories_in_source_blocks(
                         sopts.unstable_opts.ignore_directory_in_diagnostics_source_blocks.clone(),
                     );
@@ -1470,6 +1477,11 @@ fn mk_emitter(output: ErrorOutputType) -> Box<DynEmitter> {
             let short = kind.short();
             Box::new(
                 HumanEmitter::new(stderr_destination(color_config), fallback_bundle)
+                    .theme(if let HumanReadableErrorType::Unicode = kind {
+                        OutputTheme::Unicode
+                    } else {
+                        OutputTheme::Ascii
+                    })
                     .short_message(short),
             )
         }