cmd/scriggo: revise `README.txt` distributed with Scriggo binaries

The README.txt file distributed with Scriggo binaries is a bit outdated,
doesn't document some new features, and is also too cumbersome to
maintain.

It seems more reasonable to leave minimal documentation there and link
to the website (with the documentation) and the GitHub repository, which
contain updated and more extensive documentation.

This commit also introduces some small styling changes to the file.

Closes #1014.

cmd/scriggo: exit with status code 2 when running just `scriggo`

Currently, when 'scriggo' is run without any other
parameters/subcommands, it returns exit code 0.

This is generally not expected, as it represents an error in which
nothing has been executed.

The go command, for example, returns 2 in this case.

So this commit ensures that in this case, scriggo exits with exit code
2, which also appears to be the behavior of the Bash builtins.

This appears to be the only case in the Scriggo command where a success
status was returned when the command was called with incorrect
arguments, so it has been fixed separately in this commit; for all other
cases, see issue #1012.

scriggo: require Go 1.23 as the minimum supported version

With the release of Go 1.24, this change sets Go 1.23 as the new minimum
supported version. Additionally, the `scriggo` command is now compiled
using Go 1.24.

Co-authored-by: Gianluca Mondini <mondini@open2b.com>

internal/compiler: fix emitting of receiver in method calls

This commit fixes the emitting of method calls, where the receiver of
the method call was erroneously placed as first "general" argument in
the call stack, causing an incorrect stack shift and altering the
parameters received from the called method.

Also adds tests on this.

Fix #967.

Co-authored-by: Marco Gazerro <gazerro@open2b.com>

cmd/scriggo: add LiveReload for automatic page reloads

This change adds LiveReload functionality, enabling automatic page
reloads in the browser whenever the template code changes.

The 'scriggo serve' command now injects JavaScript into the generated
page. Using Server-Side Events (SSE), it triggers a page reload whenever
the page’s template files change.

LiveReload can be disabled with the --disable-livereload flag.

scriggo/test: update `github.com/rogpeppe/go-internal` to v1.13.1

native,internal/runtime: introduce the `Env.MarkdownConverter` method

This change introduces the 'Env.MarkdownConverter' method, which returns
the Markdown converter provided to the BuildTemplate function.

scriggo: update the configuration of GoReleaser (`.goreleaser.yml`)

all: use the syntax `//go:build` instead of `// +build`

This commit removes the line syntax:

	// +build

by using just the syntax:

	//go:build

which is supported starting from Go 1.17 and has been considered
deprecated starting from Go 1.18.

Note that, in tests, the old syntax is kept. See #956.

builtin: fix invalid behavior of function 'HasSuffix'

Also add tests on it and on function 'HasPrefix'.