stdlib: temporarily add AST pattern matching

[bung87]

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.

[zerbina]

I've update the PR message and title. The issues with the previous one were as follows:

• the summary was missing a short description of what the module does and why it's added to the experimental standard library
• the reason provided in the details section was not something concrete ("it may be useful")

Since it's intended for use by the LSP during the latter's development, I've explicitly mentioned the addition's uncertain future.

[zerbina]

/merge

[github-actions]

Merge requested by: @zerbina

Contents after the first section break of the PR description has been removed and preserved below:

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