Add POSIX parameter expansions and initialization

[Neved4]

For validation, we could use something like grok.sh or https://stackoverflow.com/a/67261832.

Run code as nsh -c '<code>', e.g.: nsh -c 'var='.

Features

• [ ] Empty variable assignment

  #!/bin/sh
  var=

  nsh: parse error:  --> 2:5
  |
  2 | var=␊
  |    ^---
  |
  = expected literal_span, tilde_span, any_string_span, any_char_span, command_span, proc_subst_direction, backtick_span, expr_span, param_ex_span, param_span, double_quoted_span, single_quoted_span, or initializer

Notice how ␊ is shown when there's a newline.

• [ ] Default value substitution with whitespace

  echo "${var:- }"

  nsh: parse error:  --> 1:14
  |
  1 | echo "${var:- }"
  |              ^---
  |
  = expected word

• [ ] Default value initialization

  : "${var:=def}"

  nsh: command not found `:'

• [ ] Get name without extension

  echo ${FILENAME%.*}

  nsh: parse error:  --> 1:16
  |
  1 | echo ${FILENAME%.*}
  |                ^---
  |
  = expected param_op

• [ ] Get extension

  echo ${FILENAME##*.}

  nsh: parse error:  --> 1:16
  |
  1 | echo ${FILENAME##*.}
  |                ^---
  |
  = expected param_op

• [ ] Get directory name

  echo ${PATHNAME%/*}

  nsh: parse error:  --> 1:16
  |
  1 | echo ${PATHNAME%/*}
  |                ^---
  |
  = expected param_op

• [ ] Get file name

  echo ${PATHNAME##*/}

  nsh: parse error:  --> 1:16
  |
  1 | echo ${PATHNAME##*/}
  |                ^---
  |
  = expected param_op

---

For reference, here is the expansion table:	parameter expansion	var set	var empty	var unset
${var-default}	$var	""	default
${var=default}	$var	""	default
${var?default}	$var	""	exit 127
${var+default}	default	default	""
${var:-default}	$var	default	default
${var:=default}	$var	default	default
${var:?default}	$var	exit 127	exit 127
${var:+default}	default	""	""

For POSIX parameter expansion, see: POSIX.1-2017: Parameter Expansion.
For other applications of parameter expansion, see: The Bash Hackers Wiki: Parameter Expansion.
For a more historical reference, see: The Traditional Bourne Shell Family.