Substring 'not' cannot be used in expressions
(The following tests are done in web IDE)

Suppose we want to parse a UTF-16 string with character length info described as `u2`. The `.ksy` is defined like this.

```yaml
meta:
  id: test_bug
  file-extension: .bin
  endian: le
seq:
  - id: note_len
    type: u2
  - id: note
    type: str
    size: note_len * 2
    encoding: UTF-16LE
```
Unfortunately, it raises exception as below:
```
Parse error: undefined
Call stack: Error
    at $c_Lio_kaitai_struct_precompile_ErrorInInput.fillInStackTrace__jl_Throwable (https://ide.kaitai.io/lib/_npm/kaitai-struct-compiler/kaitai-struct-compiler.js:20102:14)
    at $c_Lio_kaitai_struct_precompile_ErrorInInput.init___T__jl_Throwable (https://ide.kaitai.io/lib/_npm/kaitai-struct-compiler/kaitai-struct-compiler.js:20124:8)
    at $c_Lio_kaitai_struct_precompile_ErrorInInput.init___jl_Throwable__sci_List__s_Option (https://ide.kaitai.io/lib/_npm/kaitai-struct-compiler/kaitai-struct-compiler.js:55387:52)
    at $c_Lio_kaitai_struct_precompile_TypeValidator.run__V (https://ide.kaitai.io/lib/_npm/kaitai-struct-compiler/kaitai-struct-compiler.js:8842:70)
    at $c_Lio_kaitai_struct_Main$.precompile__Lio_kaitai_struct_format_ClassSpecs__Lio_kaitai_struct_format_ClassSpec__Lio_kaitai_struct_RuntimeConfig__V (https://ide.kaitai.io/lib/_npm/kaitai-struct-compiler/kaitai-struct-compiler.js:3552:156)
    at https://ide.kaitai.io/lib/_npm/kaitai-struct-compiler/kaitai-struct-compiler.js:3506:38
    at $c_sjsr_AnonFunction1.apply__O__O (https://ide.kaitai.io/lib/_npm/kaitai-struct-compiler/kaitai-struct-compiler.js:28035:23)
    at $c_s_util_Success.map__F1__s_util_Try (https://ide.kaitai.io/lib/_npm/kaitai-struct-compiler/kaitai-struct-compiler.js:46244:47)
    at https://ide.kaitai.io/lib/_npm/kaitai-struct-compiler/kaitai-struct-compiler.js:13405:18
    at $c_sjsr_AnonFunction1.apply__O__O (https://ide.kaitai.io/lib/_npm/kaitai-struct-compiler/kaitai-struct-compiler.js:28035:23)
    at $f_s_concurrent_impl_Promise__liftedTree1$1__ps_concurrent_impl_Promise__F1__s_util_Try__s_util_Try (https://ide.kaitai.io/lib/_npm/kaitai-struct-compiler/kaitai-struct-compiler.js:26941:31)
    at https://ide.kaitai.io/lib/_npm/kaitai-struct-compiler/kaitai-struct-compiler.js:26933:22
    at $c_sjsr_AnonFunction1.apply__O__O (https://ide.kaitai.io/lib/_npm/kaitai-struct-compiler/kaitai-struct-compiler.js:28035:23)
    at $c_s_concurrent_impl_CallbackRunnable.run__V (https://ide.kaitai.io/lib/_npm/kaitai-struct-compiler/kaitai-struct-compiler.js:26813:23)
    at $c_sjs_concurrent_QueueExecutionContext$PromisesExecutionContext.scala$scalajs$concurrent$QueueExecutionContext$PromisesExecutionContext$$$anonfun$execute$2__sr_BoxedUnit__jl_Runnable__sjs_js_$bar (https://ide.kaitai.io/lib/_npm/kaitai-struct-compiler/kaitai-struct-compiler.js:33041:16)
    at https://ide.kaitai.io/lib/_npm/kaitai-struct-compiler/kaitai-struct-compiler.js:33058:24 io.kaitai.struct.precompile.ErrorInInput: test_bug: /seq/1/size: unable to access 'e_len' in test_bug context
```
This is because `not` as the substring of `note_len` is treated as a keyword in expressions, even if it should be part of an identifier.

It should not be treated as a separated keyword to avoid confusing.

A little test file is attached as below (zipped since .bin is not allowed to be uploaded).
[TestBin.zip](https://github.com/kaitai-io/kaitai_struct/files/3038135/TestBin.zip)

 