SourceFile text does not include BOM

@andrewbranch 已经在做这个了。

开始于 2026年7月7日。

## Steps to reproduce

The `sourceFile.{text,getFullText(),getText()}` properties return a string that does not include the BOM from the original file, however the node positions are relative to the string with the BOM. This means the node positions and their text do not align.

```ts
import * as ts from "@typescript/native-preview/unstable/ast";
import { createVirtualFileSystem } from "@typescript/native-preview/unstable/fs";
import { API } from "@typescript/native-preview/unstable/sync";

const UTF8_BOM = "\uFEFF";
const INPUT = `${UTF8_BOM}let answer:number=42;`;

const fs = createVirtualFileSystem({
"/tsconfig.json": JSON.stringify({ files: ["/input.ts"] }),
"/input.ts": INPUT,
});
const api = new API({
cwd: "/",
fs,
});
const snap = api.updateSnapshot({ openProject: "/tsconfig.json" });
const ast = snap.getProject("/tsconfig.json")!.program.getSourceFile("/input.ts")!;

walk(ast);

function walk(node: ts.Node) {
const start = node.getFullStart();
const end = node.getEnd();
const src = node.getSourceFile().getText();
console.log({ start, end, text: src.slice(start, end), text2: node.getFullText() });
node.forEachChild(walk);
}
```

## Behavior with `typescript@6.0`

```ts
import * as ts from "typescript";

const UTF8_BOM = "\uFEFF";
const INPUT = `${UTF8_BOM}let answer:number=42;`;

const ast = ts.createSourceFile("input.ts", INPUT, ts.ScriptTarget.ESNext, /* setParentNodes */true);

walk(ast);

function walk(node: ts.Node) {
const start = node.getFullStart();
const end = node.getEnd();
const fullText = node.getSourceFile().getFullText();
console.log({ start, end, text: fullText.slice(start, end), text2: node.getFullText() });
node.forEachChild(walk);
}
```

Prints:

```
...
{ start: 4, end: 11, text: ' answer', text2: ' answer' }
{ start: 12, end: 18, text: 'number', text2: 'number' }
{ start: 19, end: 21, text: '42', text2: '42' }
{ start: 22, end: 22, text: '', text2: '' }
```

## Behavior with `tsgo`

```
...
{ start: 4, end: 11, text: 'answer:', text2: 'answer:' }
{ start: 12, end: 18, text: 'umber=', text2: 'umber=' }
{ start: 19, end: 21, text: '2;', text2: '2;' }
{ start: 22, end: 22, text: '', text2: '' }
```