Closes#4283.
Heredocs are implemented in a way that makes them feel more like a
string (and not a weird redirection, a la bash).
There are two tunables, whether the string is dedented (`<<-` vs `<<~`)
and whether it allows interpolation (quoted key vs not).
To the familiar people, this is how Ruby handles them, and I feel is the
most elegant heredoc syntax.
Unlike the oddjob that is bash, heredocs are treated exactly as normal
strings, and can be used _anywhere_ where a string can be used.
They are *required* to appear in the same order as used after a newline
is seen when parsing the sequence that the heredoc is used in.
For instance:
```sh
echo <<-doc1 <<-doc2 | blah blah
contents for doc1
doc1
contents for doc2
doc2
```
The typical nice errors are also implemented :^)
SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
Now a variable may have an optional slice (only _one_ slice), which can
also use negative indices to index from the end.
This works on both lists and strings.
The contents of the slice have the same semantics as brace expansions.
For example:
```sh
$ x=(1 2 3 4 5 6)
$ echo $x[1..3] # select indices 1, 2, 3
2 3 4
$ echo $x[3,4,1,0] # select indices 3, 4, 1, 0 (in that order)
4 5 2 1
$ x="Well Hello Friends!"
$ echo $x[5..9]
Hello
```
This commit adds a few basic variable substitution operations:
- length
Find the length of a string or a list
- length_across
Find the lengths of things inside a list
- remove_{suffix,prefix}
Remove a suffix or a prefix from all the passed values
- regex_replace
Replace all matches of a given regex with a given template
- split
Split the given string with the given delimiter (or to its
code points if the delimiter is empty)
- concat_lists
concatenates any given lists into one
Closes#4316 (the ancient version of this same feature)
