Add map literal syntax

[dain]

Description

This PR adds a simple map literal syntax to the grammar. The grammar uses a comma separated list of key/value entries separated by =>. This is similar to the syntax for a function call with named parameters.

Under the covers the literal is translated to map_from_entries.

Examples

map('x' => 1, 'y' => 2);
map(1.1 => 'a', 2.2 => 'b');

Release notes

(X) Release notes are required, with the following suggested text:

## Section
* Add map literal syntax. For example, `map(1.1 => 'a', 2.2 => 'b')` ({issue}`issuenumber`)

[martint]

The proposed syntax is too similar to a function call, but has very subtle differences (keys in place of argument names, arbitrary numbers of arguments) that make it confusing and potentially ambiguous.

For instance, consider the following case:

map(a => 1, b => 2)

vs

custom_map(a => 1, b => 2)

where map is the proposed map literal syntax, and custom_map is a user defined function. In the first case, a and b are references that must come from an outer scope (e.g., column names). In the second case, they are argument names that values 1 and 2 get bound to. They look visually very similar, but have vastly different semantics.

For map literals, I'd prefer to go more traditional syntax, such as {key1 : value1, key2 : value2, ... } or {key1 => value1, key2 => value2, ...} (as a side note, we may want to consider the same for arrays to be able to omit the array prefix in array[1,2,3] at some point).

See previous discussions on this topic: prestodb/presto#5065

[electrum]

Let's use the {key1 : value1, key2 : value2, ... } syntax, which matches JSON, JavaScript, Python, Ruby, Snowflake, CQL, etc.