Aspirational scopes for `LET` and `LAMBDA`

[michaelblyons]

Don't work too hard on this. I think it's possible with branching, but there may be other things to look at first that are lower-hanging fruit.

=LAMBDA(param1, param2, TEXT(param1, param2))
'^^^^^^ keyword.declaration.function.inline
'       ^^^^^^^^^^^^^^ meta.function.inline.parameters
'        ^^^^^ variable.parameter
'            ^ punctuation.separator.sequence
'                ^^^^^ variable.parameter
'                     ^ punctuation.section.function.begin ?
'                      ^^^^^^^^^^^^^^^^^^^^^^ meta.function.inline.body

=LET(var1, 1, var2, 2, TEXT(var1, var1))
'^^^ keyword.declaration.variable
'    ^^^^ variable.other.readwrite
'        ^ keyword.operator.assignment
'           ^ punctuation.separator.sequence
'             ^^^^ variable.other.readwrite
'                 ^ keyword.operator.assignment
'                    ^ punctuation.separator.sequence