Add parser recovery for incomplete `let x = ` statement

[Centril]

As per https://github.com/FuelLabs/sway/pull/2724#pullrequestreview-1104953282, we need to recover if the user wrote e.g., let x = but did not enter a following expression, or failed to terminate the statement with ;.

This probably needs some lexer tweaks, so that we can even do an enter_delimited. Otherwise, we'll not even attempt parsing the block. However, the lexer probably also lacks necessary info to do this intelligently, if we just attempt to recover for any unclosed delimiters. So we should let the parser know where recovery was made. See https://github.com/rust-lang/rust/blob/87788097b776f8e3662f76627944230684b671bd/compiler/rustc_parse/src/parser/diagnostics.rs#L2239-L2312 for some relevant code.