[w3c/clipboard-apis] Defer OS clipboard read from clipboard.read() to ClipboardItem.getType() (PR #248)

Spec change to defer OS clipboard reads from `clipboard.read()` to
  `ClipboardItem.getType()`.

  Previously, `clipboard.read()` eagerly fetched all clipboard payload bytes from
  the OS at call time. This change specifies that `clipboard.read()` enumerates
  only the available format names (metadata), and the actual OS data read is
  deferred to when the author calls `getType()` on the resulting `ClipboardItem`.

  Specific changes:
  - `clipboard.read()`: initializes each representation's data to a new unresolved
    promise instead of eagerly resolving it with OS clipboard bytes. Resolves the
    existing Issue: "It should be possible to read the data asynchronously from
    the system clipboard after the author calls getType".
  - `ClipboardItem.getType()`: adds a deferred OS read block that runs in parallel,
    reads bytes via the `os specific well-known format` algorithm, applies
    sanitization (same rules as before), and queues resolution on the clipboard
    task source per Infra conventions.


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-- Commit Summary --

  * Defer OS clipboard read from clipboard.read() to ClipboardItem.getType()

-- File Changes --

    M index.bs (58)

-- Patch Links --

https://github.com/w3c/clipboard-apis/pull/248.patch?email_source=notifications&email_token=ACLYVMPJFW6T6BYVOEEI37T4SVD3DA5CNFSNUABEM5UWIORPF5TWS5BNNB2WEL2QOVWGYUTFOF2WK43UF4ZTINJSGE3DGNZZHGTHEZLBONXW5KTTOVRHGY3SNFRGKZFFMV3GK3TUVZYHEX3QMF2GG2C7MNWGSY3Lhttps://github.com/w3c/clipboard-apis/pull/248.diff?email_source=notifications&email_token=ACLYVMN23A7LFFYRDJKHBHL4SVD3DA5CNFSNUABEM5UWIORPF5TWS5BNNB2WEL2QOVWGYUTFOF2WK43UF4ZTINJSGE3DGNZZHGTHEZLBONXW5KTTOVRHGY3SNFRGKZFFMV3GK3TUVVYHEX3ENFTGMX3DNRUWG2Y
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