Add "" container block

[cromo]

Since the current text entry mechanism only allows adding or removing text from the end, so realizing you forgot a quote at the beginning of a long string is an annoying time loss. This would allow multiple blocks to be strung together with a single container block, making it easier to re-arrange parts of a string and preventing having to retype everything to insert the initial quote. This could use a similar (or maybe even the same?) mechanism as regular container blocks introduced in #15.