Make default npm project path configurable via taskfile variable

A repository might contain multiple npm-managed projects. For this reason, the appropriate npm-managed tasks have a
parameter environment variable that allows it to be configured for an arbitrary path, with the "Check npm" workflow
having a job matrix of paths to pass.

Generally, even if there are multiple npm-managed projects, there will be one primary project that is most often the
target of contributor operation. Since it would be inconvenient for the contributor to pass the environment variable
path every time they want to run a task for that primary project, the tasks are configured to have a default path which
is used if the variable is not defined by the user.

Since the primary npm-managed project would typically be in the root of the repository, the default value is set to the
root. In the case where the primary npm-managed project is not in the root of the repository, the template installer
will need to adjust this. Previously the default was hard coded in each individual task. The template will be made
easier to install by defining the default in a single place via a friendly taskfile variable, following the convention
already established by the templates for Go-based projects.

Author: per1234

Taskfile.yml

@@ -5,6 +5,9 @@ includes:
   dist: ./DistTasks.yml
 
 vars:
+  # Source: https://github.com/arduino/tooling-project-assets/blob/main/workflow-templates/assets/npm-task/Taskfile.yml
+  # Path of the project's primary npm-managed project:
+  DEFAULT_NPM_PROJECT_PATH: .
   # Source: https://github.com/arduino/tooling-project-assets/blob/main/workflow-templates/assets/release-go-task/Taskfile.yml
   PROJECT_NAME: "arduino-lint"
   DIST_DIR: "dist"
@@ -155,6 +158,8 @@ tasks:
       WORKFLOWS_DATA_PATH: "./.github/workflows/*.{yml,yaml}"
     deps:
       - task: npm:install-deps
+        vars:
+          PROJECT_PATH: .
     cmds:
       - |
         wget \
@@ -248,6 +253,8 @@ tasks:
     desc: Format all supported files with Prettier
     deps:
       - task: npm:install-deps
+        vars:
+          PROJECT_PATH: .
     cmds:
       - |
         npx \
@@ -412,6 +419,8 @@ tasks:
     deps:
       - task: docs:generate
       - task: npm:install-deps
+        vars:
+          PROJECT_PATH: .
     cmds:
       - |
         npx \
@@ -423,6 +432,8 @@ tasks:
     desc: Automatically correct linting violations in Markdown files where possible
     deps:
       - task: npm:install-deps
+        vars:
+          PROJECT_PATH: .
     cmds:
       - |
         npx \
@@ -435,6 +446,8 @@ tasks:
     desc: Check for problems in Markdown files
     deps:
       - task: npm:install-deps
+        vars:
+          PROJECT_PATH: .
     cmds:
       - |
         npx \
@@ -447,7 +460,7 @@ tasks:
   npm:install-deps:
     desc: Install dependencies managed by npm
     dir: |
-      "{{default "./" .PROJECT_PATH}}"
+      "{{default .DEFAULT_NPM_PROJECT_PATH .PROJECT_PATH}}"
     cmds:
       - npm install
 
@@ -498,7 +511,7 @@ tasks:
       STYLELINTRC_SCHEMA_PATH:
         sh: task utility:mktemp-file TEMPLATE="stylelintrc-schema-XXXXXXXXXX.json"
       INSTANCE_PATH: >-
-        {{default "." .PROJECT_PATH}}/package.json
+        {{default .DEFAULT_NPM_PROJECT_PATH .PROJECT_PATH}}/package.json
       PROJECT_FOLDER:
         sh: pwd
       WORKING_FOLDER: