previous character had not been set and was 0.

A comment in bash is defined as "[...] a word beginning with # causes
that word and all remaining characters on that line to be ignored".

A word is defined as "a sequence of characters considered as a single
unit by the shell";  and there is a set of metacharacters defined as
"a character that, when unquoted, separates words.  One of the
following:  | & ; ( ) < > space tab".

In practice, "foo#bar" is one single word, not "foo" followed by a
comment.  Trickier, "foo\;#bar" is also a single word, but "foo;bar"
are 2 words and a control character.

So, fix the Bash lexer to check whether the character preceding the
hash sign to be either a metacharacter or part of a word.
A maybe better fix would be to understand the Bash conception of a
word, and analyze those, but it would require a large rewrite.

Bugs #1515556, #3008483, #3512208, #3515392.
From Kein-Hong Man.

From Kein-Hong Man.
- fixed SCE_PL_DEFAULT constant reported by Matthew Brush
- backtracks to previous line if necessary to resolve refresh issues for
multiline segments (this SF bug 3526563)
- properly refreshes HERE document body and code following it when HERE
delimiter is changed
- HERE delimiter with no ending quote is properly highlighted as an error,
this did not previously work
- leading spaces highlighted properly in quoted HERE delimiters
- recognizes '' and "" HERE delimiters to match blank lines

Dropped declaration not needed in LexCPP.