stdlib: temporarily add AST pattern matching (#882)

## Summary

Add the `ast_pattern_matching` module to the experimental standard
library. It implements pattern matching for `NimNode` AST and is copied
from https://github.com/krux02/ast-pattern-matching.

The first iteration of the language-server protocol (LSP) implementation
is going to use the pattern matching, but it's likely that the module
will be removed from the standard library again.

## Details

A copy of the `ast-pattern-matching` implementation already existed
within the `tests` directory, where it is used by the `tastpec.nim`
test. The module is moved to the `lib/experimental` directory and the
import is adjusted.

tests/lang_syntax/ast_pattern_matching.nim → lib/experimental/ast_pattern_matching.nim

@@ -1,5 +1,5 @@
-# this is a copy paste implementation of github.com/krux02/ast_pattern_matching
-# Please provide bugfixes upstream first before adding them here.
+## Implements pattern matching for ``NimNode`` AST. This module is copied
+## from https://github.com/krux02/ast-pattern-matching.
 
 import macros, strutils, tables
 

tests/lang_syntax/astspec/tastspec.nim

@@ -5,7 +5,7 @@ joinable: false
 
 # this test should ensure that the AST doesn't change slightly without it getting noticed.
 
-import ../ast_pattern_matching
+import experimental/ast_pattern_matching
 
 template expectNimNode(arg: untyped): NimNode = arg
   ## This template here is just to be injected by `myquote`, so that