Clean up regexp in `lua-font-lock-keywords`

Remove superfluous use of surrounding "ws" symbol in the regexp for
detecting the labels and "goto" calls.

Ensure that the regexp for font-locking "goto" calls checks for symbol
boundaries (avoids font-locking "bar" in the following "JUNKgoto bar").

Adapt unit tests to new regexp's
Fix algorithm use to calculate indentation of labels

Add the character ":" into the variable `lua-operator-class`. This
allows the cleanup of some code in functions
`lua-last-token-continues-p` and `lua-first-token-continues-p`.