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>

src/common/parser/EscapeSequenceParser.test.ts

@@ -6,11 +6,11 @@
 import { IParsingState, IParams, ParamsArray, IOscParser, IOscHandler, OscFallbackHandlerType, IFunctionIdentifier, IParserStackState, ParserStackType } from 'common/parser/Types';
 import { EscapeSequenceParser, TransitionTable, VT500_TRANSITION_TABLE } from 'common/parser/EscapeSequenceParser';
 import { assert } from 'chai';
-import { StringToUtf32, stringFromCodePoint, utf32ToString } from 'common/input/TextDecoder';
+import { StringToUtf32, stringFromCodePoint, utf32ToString } from 'common/encoding/TextDecoder';
 import { ParserState } from 'common/parser/Constants';
 import { Params } from 'common/parser/Params';
 import { OscHandler } from 'common/parser/OscParser';
-import { IDisposable } from 'common/Types';
+import { IDisposable } from 'common/services/Types';
 import { DcsHandler } from 'common/parser/DcsParser';
 import { ApcHandler } from 'common/parser/ApcParser';