feat: add expr-syntax rules and expr-tests

[shruti2522]

1. Does this PR affect any open issues?(Y/N) and add issue references (e.g. "fix #123", "re #123".):

• N
• Y
  re [PreTest] [LFX] Syntax and test case supplementation for expression nodes #8

3. Provide a description of the PR(e.g. more details, effects, motivations or doc link):

added expr:

• null_coalesce operator (expr1 or expr2).
• smoke_expr
• nonstring_literal_expr
• string_literal_expr
• number_bin_suffix_expr
• config_expr
• bracket_expr
• braces_expr

added tests for:

• null coalesce expression
• paren expression
• boolean expression
• not operator
• lambda expression
• comparison operators
• smoke expr
• nonstring_literal_expr
• string_literal_expr
• number_bin_suffix_expr
• config_expr
• bracket_expr
• braces_expr

[Peefy]

LGTM