better comment parsing in rules files

[simonmichael]

In .rules files, parsing of # comment lines and same-line comments could be improved. Eg

• Within pattern lines of an if rule, a # will be interpreted as part of the pattern, where you might expect it to disable that pattern. (And hledger-vscode highlights it as if it was a comment.)
• Within an if rule's body (assignments), a # will break parsing.