Rename str2var to str_to_var and similar

Affects the Math class, a good chunk of the audio code, and a lot of other miscellaneous classes, too.

- `var2str` -> `var_to_str`
- `str2var` -> `str_to_var`
- `bytes2var` -> `bytes_to_var`
- `bytes2var_with_objects` -> `bytes_to_var_with_objects`
- `var2bytes` -> `var_to_bytes`
- `var2bytes_with_objects` -> `var_to_bytes_with_objects`
- `linear2db` -> `linear_to_db`
- `db2linear` -> `db_to_linear`
- `deg2rad` -> `deg_to_rad`
- `rad2deg` -> `rad_to_deg`

- `dict2inst` -> `dict_to_inst`
- `inst2dict` -> `inst_to_dict`

doc/classes/Vector2.xml

@@ -304,7 +304,7 @@
 			<return type="Vector2" />
 			<param index="0" name="angle" type="float" />
 			<description>
-				Returns the vector rotated by [param angle] (in radians). See also [method @GlobalScope.deg2rad].
+				Returns the vector rotated by [param angle] (in radians). See also [method @GlobalScope.deg_to_rad].
 			</description>
 		</method>
 		<method name="round" qualifiers="const">