Add support for const object members (#678)

This adds support for adding the `const` modifier to object members.

Such object members are also required to have the `local` modifier applied.

This follows SPICE-0011.

pkl-core/src/test/files/LanguageSnippetTests/input-helper/modules/Birds2.pkl

@@ -0,0 +1,5 @@
+module Birds2
+
+abstract class Bird
+
+bird: Bird

pkl-core/src/test/files/LanguageSnippetTests/input/basic/constModifier4.pkl

@@ -0,0 +1,15 @@
+const baz = 15
+
+foo {
+  const local bar = baz
+
+  const local bar2 = biz()
+
+  const local function biz() = baz
+
+  qux = bar
+
+  corge = biz()
+
+  corge2 = bar2
+}

pkl-core/src/test/files/LanguageSnippetTests/input/errors/const/constLocalAmendModule.pkl

@@ -0,0 +1,9 @@
+amends ".../input-helper/modules/Birds2.pkl"
+
+const local myBirdName = "Birdy"
+
+local class MyBird extends Bird {
+  name: String = myBirdName
+}
+
+bird = new MyBird {}

pkl-core/src/test/files/LanguageSnippetTests/input/errors/const/constLocalMethod.pkl

@@ -0,0 +1,7 @@
+bar = 15
+
+obj {
+  const local function foo() = bar
+  
+  res = foo()
+}

pkl-core/src/test/files/LanguageSnippetTests/input/errors/const/constLocalProperty.pkl

@@ -0,0 +1,5 @@
+foo {
+  res1 = 15
+  const local qux = res1
+  res2 = qux
+}

pkl-core/src/test/files/LanguageSnippetTests/output/basic/constModifier4.pcf

@@ -0,0 +1,6 @@
+baz = 15
+foo {
+  qux = 15
+  corge = 15
+  corge2 = 15
+}

pkl-core/src/test/files/LanguageSnippetTests/output/errors/const/constLocalAmendModule.pcf

@@ -0,0 +1,3 @@
+bird {
+  name = "Birdy"
+}

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

@@ -0,0 +1,18 @@
+–– Pkl Error ––
+Cannot reference property `bar` from here because it is not `const`.
+
+x | const local function foo() = bar
+                                 ^^^
+at constLocalMethod#obj.foo (file:///$snippetsDir/input/errors/const/constLocalMethod.pkl)
+
+To fix, do either of:
+1. Add modifier `const` to property `bar`
+2. Self-import this module, and reference this property from the import.
+
+x | res = foo()
+          ^^^^^
+at constLocalMethod#obj.res (file:///$snippetsDir/input/errors/const/constLocalMethod.pkl)
+
+xxx | text = renderer.renderDocument(value)
+             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+at pkl.base#Module.output.text (pkl:base)

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

@@ -0,0 +1,18 @@
+–– Pkl Error ––
+Cannot reference property `res1` from here because it is not `const`.
+
+x | const local qux = res1
+                      ^^^^
+at constLocalProperty#foo.qux (file:///$snippetsDir/input/errors/const/constLocalProperty.pkl)
+
+To fix, do either of:
+1. Add modifier `const` to property `res1`
+2. Self-import this module, and reference this property from the import.
+
+x | res2 = qux
+           ^^^
+at constLocalProperty#foo.res2 (file:///$snippetsDir/input/errors/const/constLocalProperty.pkl)
+
+xxx | text = renderer.renderDocument(value)
+             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+at pkl.base#Module.output.text (pkl:base)

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

@@ -1,5 +1,5 @@
 –– Pkl Error ––
-Modifier `const` is not applicable to object members.
+Modifier `const` can only be applied to object members that are also `local`.
 
 x | const foo = 10
     ^^^^^