TermInterface implementation incomplete

[karlwessel]

The implementation for TermInterface is missing the isexpr method. It got removed in 45e5b14#diff-525588e68b2421901be164965272940effa093de733a3fd36c4b9a4344b8c20cL623

MWE:

julia> using SymbolicUtils

julia> @syms x
(x,)

julia> iscall(sin(x))
true

julia> isexpr(sin(x))
ERROR: UndefVarError: `isexpr` not defined in `Main`
Suggestion: check for spelling errors or missing imports.
Hint: a global variable of this name also exists in Base.
Stacktrace:
 [1] top-level scope
   @ REPL[6]:1

julia> using TermInterface

julia> isexpr(sin(x))
false