Produce more helpful output when module output is overwritten (#716)

Fix a bug where overwriting `output` causes a PklBugException.

This now produces a more helpful message pointing to the actual problem.

Co-authored-by: translatenix <119817707+translatenix@users.noreply.github.com>

pkl-core/src/test/files/LanguageSnippetTests/input/errors/invalidOutput1.pkl

@@ -0,0 +1 @@
+output: String = "abc"

pkl-core/src/test/files/LanguageSnippetTests/input/errors/invalidOutput2.pkl

@@ -0,0 +1,3 @@
+class Test {}
+
+output: Test = new {}

pkl-core/src/test/files/LanguageSnippetTests/input/errors/invalidOutput3.pkl

@@ -0,0 +1 @@
+output = null

pkl-core/src/test/files/LanguageSnippetTests/output/errors/invalidOutput1.err

@@ -0,0 +1,6 @@
+–– Pkl Error ––
+Expected `output` of module `file:///$snippetsDir/input/errors/invalidOutput1.pkl` to be of type `ModuleOutput`, but got type `String`.
+
+x | output: String = "abc"
+                     ^^^^^
+at output (file:///$snippetsDir/input/errors/invalidOutput1.pkl)

pkl-core/src/test/files/LanguageSnippetTests/output/errors/invalidOutput2.err

@@ -0,0 +1,6 @@
+–– Pkl Error ––
+Expected `output` of module `file:///$snippetsDir/input/errors/invalidOutput2.pkl` to be of type `ModuleOutput`, but got type `invalidOutput2#Test`.
+
+x | output: Test = new {}
+                   ^^^^^^
+at output (file:///$snippetsDir/input/errors/invalidOutput2.pkl)

pkl-core/src/test/files/LanguageSnippetTests/output/errors/invalidOutput3.err

@@ -0,0 +1,10 @@
+–– Pkl Error ––
+Expected value of type `ModuleOutput`, but got `null`.
+
+xx | hidden output: ModuleOutput = new {
+                    ^^^^^^^^^^^^
+at pkl.base#Module.output (pkl:base)
+
+x | output = null
+             ^^^^
+at invalidOutput3#output (file:///$snippetsDir/input/errors/invalidOutput3.pkl)