runtime(javascript): fix a few issues with syntax higlighting

runtime(javascript): fix a few issues with syntax higlighting

It addresses the following issues:

- Fix highlight of let and var javascript keywords

  According to runtime/doc/syntax.txt, Identifier is for variable names.
  let/var are not variable names, they are keywords

- Add highlighting for "from" keyword in javascript

- Fix highlight of function keyword in javascript

  According to docs, Function is for function names, so the function
  keyword should just be Keyword.

- Fix highlight of static keyword in javascript

  According to vim docs: StorageClass static, register, volatile, etc.

closes: #15480

Signed-off-by: Tobiasz Laskowski <tobil4sk@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>