Add "tea" argument to USES=tcl to facilitate porting of TEA-based extensions.

Quoting https://tcl.tk/doc/tea:

"The Tcl Extension Architecture, or TEA, by John Ousterhout and others, is a
set of guidelines and techniques for the distribution, configuration,
compilation, and installation of Tcl extensions. TEA also provides a set of
utilities that operate accordingly. Many Tcl extensions leverage the TEA
utilities, which are designed to be easily customizable."

To facilitate the porting of TEA-based extensions, I have added the new "tea"
argument to USES=tcl. This argument prepares the autoconf environment by
setting GNU_CONFIGURE and sets some commonly used CONFIGURE_ARGS. Also, it
tries to figure out the correct extension name, sets TCL_PKG accordingly, and
adds it to PLIST_SUB.

I have modified a few ports to take advantage of this. More will come.