Insert a dropped attachment above the block when the caret is at its start

[ikraamg]

Dropping a file with the caret at the very start of a heading throws Lexical invariant #66 ("Expected node to have a parent") and the file is lost. The editor is left inconsistent, so a Backspace straight after throws invariant #75.

RangeSelection.insertNodes keeps a reference to the caret's block while it calls insertParagraph(). HeadingNode.insertNewAfter replaces that block with a paragraph when the caret is at offset 0, so insertNodes goes on to insert after a node that has already left the document.

Lexical fixed this on their side in 0.47.0, by re-resolving firstBlock when it is no longer attached. Lexxy pins ^0.44.0, which under npm's 0.x caret can never reach that. I tried the bump instead of this change: lexical 0.49.0 does fix the crash with no source change, but it fails six existing tests here:

• formatting/block_formatting.test.js > toggle nested bullet list off
• formatting/list_in_quote.test.js > backspace after clearing the first bullet's text removes the blank line
• formatting/list_in_quote.test.js > backspace on a blank first list item merges the gap instead of stranding a paragraph
• formatting/list_item_deletion.test.js > deleting empty middle list item keeps cursor in the list
• paste/paste_into_quote.test.js > pasting multi-line content into a quote started via the markdown shortcut keeps every line inside the quote
• paste/paste_into_quote.test.js > the QuoteNode transform does not re-wrap toolbar-button quote children

So this adds BlockStartNodeInserter, which places a dropped attachment above the block when the caret sits at its start rather than splitting the block around the caret. Empty blocks are left to Lexical, since they are already a usable insertion point. It puts the attachment in the same place lexical 0.47 does, so it can be dropped whenever the bump happens.

It also moves the block-decorator check out of ListItemNodeInserter into $isBlockDecoratorNode, now that two inserters need it.

• reproduces on lexical 0.38.2 through 0.46.0, driven through Contents#uploadFiles the way a document-level drop zone does
• the new test fails on main and passes with the change
• yarn lint clean, yarn test green, yarn test:browser green on chromium, firefox and webkit, apart from tables/toolbar.test.js which already fails on main