Fix forgot to make new phony make targets .PHONYs
`make basic` builds mathquill-basic{,.min}.js

...which omit LatexCommandInput.js, advancedSymbols.js, and text.js.

Also, `make test` builds mathquill-basic.js too, so that unit.html can
include it and publicapi.test.js can unit-test it, and so that
visual.html can include it.

In basicSymbols.js, right after setting `LatexCmds.backslash`,
CharCmds['\\'] defaults to LatexCmds.backslash if LatexCommandInput.js
is omitted. (Note that the order of LatexCommandInput.js vs
basicSymbols.js when concatenated doesn't matter; even if basicSymbols.js
is first, LatexCommandInput.js just overwrites CharCmds['\\'].)

Closes #290, which is about configuring typing \ to insert \backslash.
Closes #306, which is about separating out commands that can only be
typed with LatexCommandInput; now separated as advancedSymbols.js.