allow operators to procede dollar quoted strings

[proddata]

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Fixes #763
Currently string literals using dollar quoting ($js$mytext$js$) must be preceded by a whitespace character, which prevents e.g. using SET application_name=$$foo;bar$$ which is valid PostgreSQL

[amotl]

Dear @andialbrecht. We are tracking this patch from another issue we are observing on our interactive database shell. Do you see any chance to have a look at it? Thanks in advance, Andreas.

• Dollar quoted strings not handling semicolon in UPDATE statements crate/crash#427

[andialbrecht]

Sorry for the delay and thanks for the contribution!