bug#60376: 29.0.60; Standardize csharp-ts-mode's font-lock features

[Yuan Fu]

Hey Theo and Jostein,

As the complete feature freeze approaching, I think it’s a good time to
standardize the font-lock features. Below is the change I would make to
csharp-ts-mode:

- take string_interpolation out of string
- add function, variable feature
- change attribute to property
- expression is not in the list, no harm to keep it
  around, of course
- maybe add assignment feature

Feel free to correct me if I misunderstood anything. TIA!

Below is the list of standard features, for your reference:

Basic tokens:

delimiter       ,.;      (delimit things)
operator        == != || (produces a value)
bracket         []{}()
misc-punctuation  anything else

constant        true, false, null
number
keyword
comment         (includes doc-comments)
string          (includes chars and docstrings)
string-interpolation    f"text {variable}"
escape-sequence         "\n\t\\"
function                every function identifier
variable                every variable identifier
type                    every type identifier
property                a.b  <--- highlight b
key                     { a: b, c: d } <--- highlight a, c
error                   highlight parse error

Abstract features:

assignment: the LHS of an assignment (thing being assigned to), eg:

a = b    <--- highlight a
a.b = c  <--- highlight b
a[1] = d <--- highlight a

definition: the thing being defined, eg:

int a(int b) { <--- highlight a
return 0
}

int a;  <-- highlight a

struct a { <--- highlight a
int b;   <--- highlight b
}