Introduce a Plugin system, implement <py-terminal> as a plugin (#917)

This PR does two things:

1. introduce the first minimal version of a Plugin system. Plugins are subclasses of the Plugin class, and pyscript will call the relevant methods/hooks at the right time. Currently, I have added only the minimal sets of hooks which were needed for this PR.

2. Implement <py-terminal> as a plugin.

pyscriptjs/tests/integration/test_py_terminal.py

@@ -0,0 +1,134 @@
+from playwright.sync_api import expect
+
+from .support import PyScriptTest
+
+
+class TestPyTerminal(PyScriptTest):
+    def test_py_terminal(self):
+        """
+        1. <py-terminal> should redirect stdout and stderr to the DOM
+
+        2. they also go to the console as usual
+
+        3. note that the console also contains PY_COMPLETE, which is a pyodide
+           initialization message, but py-terminal doesn't. This is by design
+        """
+        self.pyscript_run(
+            """
+            <py-terminal></py-terminal>
+
+            <py-script>
+                import sys
+                print('hello world')
+                print('this goes to stderr', file=sys.stderr)
+                print('this goes to stdout')
+            </py-script>
+            """
+        )
+        term = self.page.locator("py-terminal")
+        term_lines = term.inner_text().splitlines()
+        assert term_lines == [
+            "hello world",
+            "this goes to stderr",
+            "this goes to stdout",
+        ]
+        assert self.console.log.lines == [
+            self.PY_COMPLETE,
+            "hello world",
+            "this goes to stderr",
+            "this goes to stdout",
+        ]
+
+    def test_two_terminals(self):
+        """
+        Multiple <py-terminal>s can cohexist.
+        A <py-terminal> receives only output from the moment it is added to
+        the DOM.
+        """
+        self.pyscript_run(
+            """
+            <py-terminal id="term1"></py-terminal>
+
+            <py-script>
+                import js
+                print('one')
+                term2 = js.document.createElement('py-terminal')
+                term2.id = 'term2'
+                js.document.body.append(term2)
+
+                print('two')
+                print('three')
+            </py-script>
+            """
+        )
+        term1 = self.page.locator("#term1")
+        term2 = self.page.locator("#term2")
+        term1_lines = term1.inner_text().splitlines()
+        term2_lines = term2.inner_text().splitlines()
+        assert term1_lines == ["one", "two", "three"]
+        assert term2_lines == ["two", "three"]
+
+    def test_auto_attribute(self):
+        self.pyscript_run(
+            """
+            <py-terminal auto></py-terminal>
+
+            <button id="my-button" py-onClick="print('hello world')">Click me</button>
+            """
+        )
+        term = self.page.locator("py-terminal")
+        expect(term).to_be_hidden()
+        self.page.locator("button").click()
+        expect(term).to_be_visible()
+        assert term.inner_text() == "hello world\n"
+
+    def test_config_auto(self):
+        """
+        config.terminal == "auto" is the default: a <py-terminal auto> is
+        automatically added to the page
+        """
+        self.pyscript_run(
+            """
+            <button id="my-button" py-onClick="print('hello world')">Click me</button>
+            """
+        )
+        term = self.page.locator("py-terminal")
+        expect(term).to_be_hidden()
+        assert "No <py-terminal> found, adding one" in self.console.info.text
+        #
+        self.page.locator("button").click()
+        expect(term).to_be_visible()
+        assert term.inner_text() == "hello world\n"
+
+    def test_config_true(self):
+        """
+        If we set config.terminal == true, a <py-terminal> is automatically added
+        """
+        self.pyscript_run(
+            """
+            <py-config>
+                terminal = true
+            </py-config>
+
+            <py-script>
+                print('hello world')
+            </py-script>
+            """
+        )
+        term = self.page.locator("py-terminal")
+        expect(term).to_be_visible()
+        assert term.inner_text() == "hello world\n"
+
+    def test_config_false(self):
+        """
+        If we set config.terminal == false, no <py-terminal> is added
+        """
+        self.pyscript_run(
+            """
+            <py-config>
+                terminal = false
+            </py-config>
+            """
+        )
+        term = self.page.locator("py-terminal")
+        assert term.count() == 0