Bug 1869678 - Use more idiomatic code patterns for better type inference r=robwu

These fall under 5 main categories:

 1) Declare and/or initialize all class fiels in the constructor.
    (general good practise)

 2) Use real getters and redefineGetter instead of defineLazyGetter.
    (also keeps related code closer together)

 3) When subclassing, don't override class fields with getters (or vice versa).
    microsoft/TypeScript#33509

 4) Declare and assign object literals at the same time, not separatelly.
    (don't use `let foo;` at the top of the file, use `var foo = {`)

 5) Don't re-use local variables unnecesarily with different types.
    (general good practise, local variables are "free")

Differential Revision: https://phabricator.services.mozilla.com/D196386

UltraBlame original commit: 8e768446e17cc306729e3b0f705b0285c69321cf

Author: marco-c

toolkit/components/extensions/.eslintrc.js

@@ -393,9 +393,13 @@ console
 /
 No
 using
-variables
+things
 before
+they
+'
+re
 defined
+.
 "
 no
 -
@@ -406,9 +410,112 @@ before
 define
 "
 :
+[
 "
 error
 "
+{
+allowNamedExports
+:
+true
+classes
+:
+true
+/
+/
+The
+next
+two
+being
+false
+allows
+idiomatic
+patterns
+which
+are
+more
+/
+/
+type
+-
+inference
+friendly
+.
+Functions
+are
+hoisted
+so
+this
+is
+safe
+.
+functions
+:
+false
+/
+/
+This
+flag
+is
+only
+meaningful
+for
+var
+declarations
+.
+/
+/
+When
+false
+it
+still
+disallows
+use
+-
+before
+-
+define
+in
+the
+same
+scope
+.
+/
+/
+Since
+we
+only
+allow
+var
+at
+the
+global
+scope
+this
+is
+no
+worse
+than
+/
+/
+how
+we
+currently
+declare
+an
+uninitialized
+let
+at
+the
+top
+of
+file
+.
+variables
+:
+false
+}
+]
 /
 /
 Disallow

toolkit/components/extensions/ConduitsParent.sys.mjs

@@ -3114,7 +3114,8 @@ this
 )
 ;
 }
-sender
+let
+remote
 =
 Hub
 .
@@ -3128,10 +3129,10 @@ sender
 if
 (
 !
-sender
+remote
 |
 |
-sender
+remote
 .
 actor
 !
@@ -3173,7 +3174,7 @@ Hub
 .
 recvConduitClosed
 (
-sender
+remote
 )
 ;
 }
@@ -3224,6 +3225,8 @@ actor
 this
 query
 sender
+:
+remote
 }
 )
 ;