feat: introduce MagicStringError with concise, consistent messages

[antfubot]

What & why

Error messages thrown by magic-string were ad hoc new Error / new TypeError strings with inconsistent casing and tone. This normalizes every throw site behind a single, consistent error type in the spirit of "errors worth reading".

Changes

• Add MagicStringError (exported from the package entry) — the single error type thrown everywhere, extending Error.
  • Every message is prefixed with [MagicString] so its source is obvious at a glance.
  • Messages are short, lower-case, and consistent in tone, with any how-to-fix hint folded into the one message (e.g. insert() is deprecated, use appendLeft() or prependRight()).
• Convert all throws across MagicString, Bundle, and SourceMap to MagicStringError, and unify the duplicated "content must be a string" checks.

Examples:

[MagicString] content must be a string, got object
[MagicString] cannot move a selection inside itself
[MagicString] cannot overwrite a zero-length range at 3, use appendLeft() or prependRight()
[MagicString] replaceAll() requires a global RegExp

Backward compatibility

MagicStringError extends Error, so instanceof Error and try/catch keep working. A few throw sites previously produced TypeError, but that concrete type was never documented, so consolidating on MagicStringError is not considered a breaking change. Only the human-readable message wording changes.

This PR was created with the help of an agent.