Restructure common into layered folders (issue #5963)

Reorganize src/common into platform (base, encoding, data, buffer,
parser, input), integration (services, terminal), and public API layers.

- Move core utilities to common/base/
- Move TextDecoder to common/encoding/
- Move Types, CircularList, Color, TaskQueue, SortedList to common/services/
- Move CoreTerminal, InputHandler, WindowsMode to common/terminal/
- Define ICharset in common/data/Charsets.ts (re-exported from Types)
- Update imports across src/, addons/, and test/
- Add per-layer tsconfig.json files and REARCHITECTURE.md
- Root common/tsconfig.json remains the composite build entry

Co-authored-by: Daniel Imms <Tyriar@users.noreply.github.com>

test/benchmark/DecorationService.benchmark.ts

@@ -5,7 +5,7 @@
 
 import { perfContext, before, RuntimeCase } from 'xterm-benchmark';
 import { DecorationService } from 'common/services/DecorationService';
-import { MockLogService, MockBufferService, MockOptionsService } from 'common/TestUtils.test';
+import { MockLogService, MockBufferService, MockOptionsService } from 'common/terminal/TestUtils.test';
 const enum Constants {
   COLS = 80,
   ROWS = 30,

test/benchmark/Event.benchmark.ts

@@ -4,7 +4,7 @@
  */
 
 import { perfContext, before, RuntimeCase } from 'xterm-benchmark';
-import { Emitter } from 'common/Event';
+import { Emitter } from 'common/base/Event';
 
 const ITERATIONS = 1_000_000;
 

test/benchmark/Terminal.benchmark.ts

@@ -6,7 +6,7 @@
 import { perfContext, before, ThroughputRuntimeCase } from 'xterm-benchmark';
 
 import { spawn } from 'node-pty';
-import { Utf8ToUtf32, stringFromCodePoint } from 'common/input/TextDecoder';
+import { Utf8ToUtf32, stringFromCodePoint } from 'common/encoding/TextDecoder';
 import { CoreBrowserTerminal } from 'browser/CoreBrowserTerminal';
 
 perfContext('Terminal: ls -lR /usr/lib', () => {

test/playwright/TestUtils.ts

@@ -7,7 +7,7 @@ import { Browser, JSHandle, Page } from '@playwright/test';
 import { deepStrictEqual, strictEqual } from 'assert';
 import type { IRenderDimensions as IRenderDimensionsInternal } from 'browser/renderer/shared/Types';
 import type { IRenderService } from 'browser/services/Services';
-import type { ICoreTerminal, IDisposable, IMarker } from 'common/Types';
+import type { ICoreTerminal, IDisposable, IMarker } from 'common/services/Types';
 import * as playwright from '@playwright/test';
 import { IBuffer, IBufferCell, IBufferLine, IBufferNamespace, IBufferRange, IDecoration, IDecorationOptions, IModes, IRenderDimensions, ITerminalInitOnlyOptions, ITerminalOptions, Terminal } from '@xterm/xterm';