Fix Vietnamese parsing patterns

[lovelovedokidoki]

Parsing data had a mismatch in narrow month format, causing all short format (PP, MMM, LLL) months to fail parsing (it was labeled short, which is not used in the parser for month formats).

Also, the method of parsing numbers from the end of the string instead of beginning was causing the parser to read the first number of the year instead of the month, causing incorrect month results on wide format. I took the liberty to conform all the patterns to search at the beginning of the string (removing $ and adding ^ instead), to avoid future confusion, using instead negative lookahead (?!\d) to prevent the interpreter from matching numbers partially.

[leshakoss]

🙏 🌟