Remove unused function/class names when minifying (#22492)

## Summary
- Removes unused function and class expression names when
`--minify-syntax` is enabled during bundling
- Adds `--keep-names` flag to preserve original names when minifying
- Matches esbuild's minification behavior

## Problem
When minifying with `--minify-syntax`, Bun was keeping function and
class expression names even when they were never referenced, resulting
in larger bundle sizes compared to esbuild.

**Before:**
```js
export var AB = function A() { };
// Bun output: var AB = function A() {};
// esbuild output: var AB = function() {};
```

## Solution
This PR adds logic to remove unused function and class expression names
during minification, matching esbuild's behavior. Names are only removed
when:
- `--minify-syntax` is enabled
- Bundling is enabled (not transform-only mode)
- The scope doesn't contain direct eval (which could reference the name
dynamically)
- The symbol's usage count is 0

Additionally, a `--keep-names` flag has been added to preserve original
names when desired (useful for debugging or runtime reflection).

## Testing
- Updated existing test in `bundler_minify.test.ts` 
- All transpiler tests pass
- Manually verified output matches esbuild for various cases

## Examples
```bash
# Without --keep-names (names removed)
bun build --minify-syntax input.js
# var AB = function() {}

# With --keep-names (names preserved)  
bun build --minify-syntax --keep-names input.js
# var AB = function A() {}
```

🤖 Generated with [Claude Code](https://claude.ai/code)

---------

Co-authored-by: Claude Bot <claude-bot@bun.sh>
Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>
Co-authored-by: Dylan Conway <dylan.conway567@gmail.com>

docs/bundler/index.md

@@ -733,6 +733,10 @@ Whether to enable minification. Default `false`.
 When targeting `bun`, identifiers will be minified by default.
 {% /callout %}
 
+{% callout %}
+When `minify.syntax` is enabled, unused function and class expression names are removed unless `minify.keepNames` is set to `true` or `--keep-names` flag is used.
+{% /callout %}
+
 To enable all minification options:
 
 {% codetabs group="a" %}
@@ -763,12 +767,16 @@ await Bun.build({
     whitespace: true,
     identifiers: true,
     syntax: true,
+    keepNames: false, // default
   },
 })
 ```
 
 ```bash#CLI
 $ bun build ./index.tsx --outdir ./out --minify-whitespace --minify-identifiers --minify-syntax
+
+# To preserve function and class names during minification:
+$ bun build ./index.tsx --outdir ./out --minify --keep-names
 ```
 
 {% /codetabs %}
@@ -1553,6 +1561,7 @@ interface BuildConfig {
         whitespace?: boolean;
         syntax?: boolean;
         identifiers?: boolean;
+        keepNames?: boolean;
       };
   /**
    * Ignore dead code elimination/tree-shaking annotations such as @__PURE__ and package.json