Port before_v0.60/math, before_v0.60/parsing and before_v0.60/git (#844)

This PR is part of porting all old scripts #221 and includes a set of
small modules:
- `math`
- `parsing`
- `git`

modules/git/README.md

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+# Git Scripts
+
+### Definition
+
+These scripts should be used to demonstrate how combine the power of nushell with git commands.
+
+### Aliases
+If you find some of these scripts useful you can add them as aliases in your git config file.
+
+For instance you can create a nushell script in your computer and define in your git config file an alias that call that script:
+```
+[alias]
+    bcl = !nu \"D:\\Tools\\gitalias_bcl.nu\"
+```

modules/git/git_branch_age.nu

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+# Creates a table listing the branches of a git repository and the day of the last commit
+export def "git age" [] {
+    git branch |
+    lines |
+    str substring 2.. |
+    wrap name |
+    insert last_commit {
+        get name |
+        each {
+            git show $in --no-patch --format=%as | into datetime
+        }
+    } |
+    sort-by last_commit
+}
+

modules/parsing/README.md

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+# Parsing Scripts
+
+### Definition
+
+These scripts should be used to demonstrate how to parse any file format, including `json`, `csv`, etc. Also, perhaps parsing of `external` commands. For example, on Windows, perhaps we could wrap `ipconfig.exe` so that the output was more nushell friendly.

modules/parsing/sample_andres.json

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+[
+  {
+    "name": "Brunello Cucinelli",
+    "shoes": [
+      { "name": "tasselled black low-top lace-up", "price": 1000 },
+      { "name": "tasselled green low-top lace-up", "price": 1100 },
+      { "name": "plain beige suede moccasin", "price": 950 },
+      { "name": "plain olive suede moccasin", "price": 1050 }
+    ]
+  },
+  {
+    "name": "Gucci",
+    "shoes": [
+      { "name": "red leather laced sneakers", "price": 800 },
+      { "name": "black leather laced sneakers", "price": 900 }
+    ]
+  }
+]

modules/parsing/sample_andres.nu

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+def look_for [word] {
+  open sample_andres.json |
+    flatten |
+    flatten |
+    insert comp {
+        get shoes_name |
+            split row " " |
+            enumerate | each {
+        [[idx, loc]; [$in.index, ($in.item | str index-of $word)]]
+        } | flatten
+    } |
+    flatten |
+    where comp.loc >= 0 |
+    flatten |
+    update idx { $in + 1 } |
+    reject name price loc |
+    rename nameWords targetWordIndex
+}
+
+print (look_for "leather" | to json --indent 4)
+print (look_for "low-top" | to json --indent 4)