Fix field identifiers

[stsewd]

Fields are more of an identifier rather than an expression, I have renamed it to reflect this.

[stsewd]

Found a bug with

let a = ',' .b. ','

or smaller example

let a = b.'.'

but it works if there is a space

let a = b .'.'

[stsewd]

OK, I give up for today, in vimscript the . operator kind of depends on the expression being evaluated... This is the result with this change:

let a = foo.bar. ''  " This breaks, but in vimscript it's a concatenation
let a = foo.bar.''  " This breaks, but in vimscript it's a concatenation
let a = foo.bar . ''  " This is ok
let a = foo.bar . baz  " This is ok
let a = foo.bar .baz  " This is ok
let a = foo.bar. baz  " This is marked as a field, but in vimscript it's a concatenation

Previously this was

let a = foo.bar. ''  " This is marked as a field with a string as value.
let a = foo.bar.''  " This is marked as a field with a string as value.
let a = foo.bar . ''  " This is ok
let a = foo.bar . baz  " This is ok
let a = foo.bar .baz  " This is ok
let a = foo.bar. baz  " This is marked as a field, but in vimscript it's a concatenation

I'll try to play more whit this in the following days....

[vigoux]

I am quite against that tbh, the docs specify that fields are actually expressions, thus that's what they should be.

[vigoux]

Fields are horrible to work with because the . operator is type sensitive

[stsewd]

the docs specify that fields are actually expressions

That isn't what the docs say, this is what they say

	expr8.name		entry in a |Dictionary|

Only the first part is an expression, the second part after that is a name, not an expression.

[vigoux]

Indeed you are right.

[vigoux]

But in any case I preffer having the field_expression to help quering the tree, the more we avoid repetitions, the better.

[vigoux]

I don't see the point of renaming this appart from breaking backward compatibility.