completion: only show ca65 keyword doc at correct places

[taylorplewe]

What's changing

Conditionally add the ca65 keyword CompletionItems to the completion provider, only when a ca65 keyword would actually be valid

Motivation for change

Currently the keywords are just always in the completion list, regardless of where the cursor is; this is poor quality

Background/Notes

A ca65 keyword is valid:

• if it is a control command and is the lhs operand, that is, nothing should precede it on the same line.
  • the only thing that can precede it is a label:

  sintab: .byte $40, $32

  • the only exception to this rule is .set, which must follow an identifier:

  myVar .set 0

• if it is a pseudo variable or pseudo function and is the rhs operand; i.e. it must follow an instruction or control command which requires a parameter (e.g. .if, .byte, .word, lda).
  • the one exception here is .ident, which essentially replaces itself with its arguments, creating an identifier, and thus is valid wherever an identifier would be valid:

  .ident(.concat("rice", "_patty")):
  ; is the same as:
  rice_patty:

so, put another way, there are four categories of valid places a ca65 keyword could be:

• preceded by either nothing or a label
  • all control commands except .set
• preceded by an instruction or other keyword which requires a parameter
  • all pseudo variables & pseudo functions except .ident
• preceded by an identifier
  • .set
• anywhere an identifier is valid
  • .ident