Allow non-quoted identifiers as isolation test session/step names.

Allow non-quoted identifiers as isolation test session/step names.

For no obvious reason, isolationtester has always insisted that
session and step names be written with double quotes.  This is
fairly tedious and does little for test readability, especially
since the names that people actually choose almost always look
like normal identifiers.  Hence, let's tweak the lexer to allow
SQL-like identifiers not only double-quoted strings.

(They're SQL-like, not exactly SQL, because I didn't add any
case-folding logic.  Also there's no provision for U&"..." names,
not that anyone's likely to care.)

There is one incompatibility introduced by this change: if you write
"foo""bar" with no space, that used to be taken as two identifiers,
but now it's just one identifier with an embedded quote mark.

I converted all the src/test/isolation/ specfiles to remove
unnecessary double quotes, but stopped there because my
eyes were glazing over already.

Like 741d7f104, back-patch to all supported branches, so that this
isn't a stumbling block for back-patching isolation test changes.

Discussion: https://postgr.es/m/759113.1623861959@sss.pgh.pa.us