ef: Improve parse error messages with context, caret, and field hints

[jeso-mchp]

The FRAME-SPEC parse errors are not so easy to read. This changes the parse errors to a more human friendly format:

  #Before:
  $ ef hex eth bogus
  Parse error! arg# 2 out of 3, cmd_idx = 1

  $ ef hex bogus
  Parse error! arg# 0 out of 2, cmd_idx = 0

  $ ef bogus
  Parse error! arg# 0 out of 1, cmd_idx = 0

  $ ef hex eth ipv4 badfield
  Parse error! arg# 3 out of 4, cmd_idx = 1

  # After:
  $ ef hex eth bogus
  error: 'bogus' is not a field of 'eth' or a recognized header
    hex eth bogus
            ^^^^^
    valid fields for 'eth': dmac smac et

  $ ef hex bogus
  error: 'bogus' is not a recognized header or command
    hex bogus
        ^^^^^

  $ ef bogus
  error: unexpected token 'bogus'
    bogus
    ^^^^^

  $ ef hex eth ipv4 badfield
  error: 'badfield' is not a field of 'ipv4' or a recognized header
    hex eth ipv4 badfield
                 ^^^^^^^^
    valid fields for 'ipv4': ver ihl dscp ecn len id flags offset ttl proto chksum sip dip

Replace the unhelpful "Parse error! arg# N out of M" with structured diagnostics on stderr: error headline identifying the bad token and which header it was expected in, the full command with a caret pointing to the exact token, and a list of valid fields when applicable.

A global parse_err_ctx_t side-channel records context during parsing so the top-level error reporter can produce accurate messages without changing any parser function signatures.