Introduce `Prettier` class

Motivation:

Currently we save everything, include .originalText, .tokens in options, and pass through all functions.

Idea:

Add Prettier class,

• Prettier#constructor(text, options):
  Save all original input, strip BOM, strip hashbang, strip frontMatter, etc

• Prettier#parse():
  get a ParseResult, not only ast, but also text(different with originalText, the text actually pass to parser),comments, tokens, parseService, traverse, visitorKeys etc, and save to Prettier instance.

• Prettier#attachComments():
  get AttachedComments, with attach information, stop saving to AST.

• Prettier#format():
  async version of Prettier#formatSync(), replaces Prettier#formatSync() when we are ready to turn everything into async.

• Prettier#formatSync():
  same as current prettier.formatWithCursor, we may consider split it into two steps ast-to-doc, doc-to-text.

Just some basic idea, maybe we can also have Language class

Start by tracing the current prettier.formatWithCursor flow and how originalText, tokens, and options pass through the parsing and formatting functions. Review the proposed Prettier lifecycle and ParseResult contents, then clarify the design and confirm that the completed work preserves parsing metadata, comment attachment, and sync/async formatting behavior.